I Is the amount of dark matter constant in the Universe?

[Ranku]

Is the amount of dark matter constant in the universe? Is there any evidence of matter converting into dark matter, which would increase the amount of dark matter in the universe.

 

[Orodruin]

There is no evidence suggesting the amount of dark matter to be variable. If anything, leading theories of dark matter would have it decreasing ever so slightly due to annihilation or decay into standard model particles.

 

[WWGD]

There is no evidence suggesting the amount of dark matter to be variable. If anything, leading theories of dark matter would have it decreasing ever so slightly due to annihilation or decay into standard model particles.

How is DM different from " standard" at the particle level?

 

[ohwilleke]

Assuming a dark matter particle paradigm, according to a pre-print by Yang (2015) subsequently published in Physical Review D, the lower bound on the mean lifetime of dark matter particles is 3.57×10²⁴ seconds. This is roughly 10¹⁷ years. By comparison the age of the universe is roughly 1.38×10¹⁰ years.

This means that dark matter (if it exists) is at least as stable as anything other than a proton, which has an experimentally determined mean lifetime of at least 10³⁴ years, or an electron, which is theoretically stable (just as the proton is in the Standard Model) and has an experimentally determined mean lifetime of at least 6.6×10²⁸ years.

This means that all dark matter candidates that are not perfectly stable (or at least metastable) are ruled out. Decaying dark matter and dark matter with any significant annihilation cross section are inconsistent with observation, unless there is a mechanism that generates new dark matter in equilibrium with the amount annihilated. So, there has not been a discernible decline in the inferred amount of dark matter.

Equilibrium models, with dark matter created and destroyed at identical rates, however, have grown in popularity over time as thermal freeze-out dark matter particles have seen their parameter space more and more constrained over time.

In thermal freeze-out models, dark matter particles cease to interact with other matter and attain their current velocity very early in the history of the universe, not longer after their creation. In these models, mean dark matter particle velocity is related to dark matter particle mass according to a well-established relationship. But the dark matter particle masses that would be expected given reasonable estimates of mean dark matter particle velocity are high relative to other constraints (e.g., direct dark matter detection experiments have produced extremely strict cross-section of interaction constraints in that mass range, particle accelerator experiments have not found new particles in that mass range, and there are some indications that dark matter particles exhibit some wave-like behavior to an extent which would be impossible for particles of such a high mass).

Has the amount of dark matter been constant since the big bang or is it increasing? If it is increasing, is regular matter decreasing at an equal rate?

The LambdaCDM "Standard Model of Cosmology" assumes a constant amount of dark matter in the universe after the earliest moments of the universe (with the density of the dark matter in the universe decreasing in proportion to the spatial volume of the universe), just as the model does in the case of ordinary baryonic matter.

Exactly how many moments after the Big Bang it takes for dark matter to emerge is pretty much irrelevant, as this number is much smaller (by a factor of many billions) than margin of error in our estimates of the age of the universe.

There is essentially no evidence to support an increasing amount of dark matter, although admittedly, any calculation of the amount of dark matter is model dependent, and many kind of astronomy observations can't distinguish between ordinary matter and dark matter.

Caveat

Not all lines of evidence are consistent with this analysis, however. An article in the journal Nature, Bowman (March 2018), analyzing the "21 centimeter line" in the radio spectrum finds that:

[E]ither the primordial gas was much colder than expected or the background radiation temperature was hotter than expected. Astrophysical phenomena (such as radiation from stars and stellar remnants) are unlikely to account for this discrepancy; of the proposed extensions to the standard model of cosmology and particle physics, only cooling of the gas as a result of interactions between dark matter and baryons seems to explain the observed amplitude.

In other words, this evidence contradicts the LambdaCDM model, which assumes that dark matter is "almost collisionless" and hence could not cause massive cooling through interactions between dark matter and baryons. This evidence is consistent with an early universe (post-radiation era, hundreds of millions of years after the Big Bang) that has no dark matter, but that possibility throws a wrench into other aspects of the LambdaCDM model.

This contradiction has not yet been adequately resolved. Some astrophysicists think that the 21cm line data from 2018 is due to systemic error of some kind, while others think that it is real and highly significant.

(Slightly edited and revised from my Physics Stack Exchange answer of July 31, 2018).

 

[Orodruin]

How is DM different from " standard" at the particle level?

The standard model (SM) is a well defined theory of elementary particles. It describes all known elementary particles and their interactions (except gravity). Unfortunately, the SM does not contain any viable dark matter candidate. Particle dark matter would therefore have to consist of particles that are not described by the SM, i.e., physics beyond the SM.

 

[ohwilleke]

How is DM different from " standard" at the particle level?

Primarily because it is assumed to be nearly collisionless and interacting almost exclusively via gravity, while lacking at least electromagnetic and strong force interactions, and having a cross section of interaction which is much weaker than the weak force.

But the Standard Model of Particle Physics contains no such particles. Quarks have electromagnetic charge and strong and weak force interactions of standard strength. Charged leptons have electromagnetic charge and weak force interactions of standard strength. Neutrinos have standard strength weak force interactions, and we know from neutrino telescope observations that there aren't nearly enough of them in the universe to account for the inferred total mass of dark matter particles). Photons move too fast and are easily detected. Gluons have strong force interactions and are confined to strong force bound composite hadrons at temperatures energies present in outer space; they are deconfined only at temperatures found shortly after the Big Bang. W+ and W- bosons have electromagnetic and standard strength weak force interactions and are too short lived. Z bosons have standard strength weak force interactions and are too short lived. Higgs bosons are too short lived and have standard model Higgs field interactions with quarks and leptons. All known (or theoretically possible) composite particles made directly from Standard Model particle bound by the strong force (hadrons) are too short lived, have the wrong range of masses, and in many cases, have electromagnetic charge. Of course the exceptions in the Standard Model are protons and bound neutrons, which we call "ordinary matter" or "baryonic matter" (since protons and neutrons are the kind of hadron called baryons), but astronomers have quantified how many protons and neutrons exist, and this is an insufficient amount of mass to explain dark matter phenomena.

Direct dark matter searches affirm these nearly sterile inferred properties in a dark matter particle model. See, e.g., this chart:

From J. Aalbers, et al., "Dark Matter Search Results from 4.2 Tonne-Years of Exposure of the LUX-ZEPLIN (LZ) Experiment" arXiv:2410.17036 (October 22, 2024).

The cross-section of interaction of a neutrino with a nucleon is a little less than 10^-38 cm². The maximum cross-section of dark matter particles with masses from 9 GeV to 10,000 GeV in light of the latest Lux-Zeplin data is 10^-45 cm² (i.e. ten million times smaller), and for masses of 11 GeV to 150 GeV it is 10^-47 cm² (i.e. a billion times smaller). This is far below the threshold for dark matter candidates such as Higgs portal, Z portal, W portal, and millicharged dark matter candidates. Those thresholds were already passed in 2018.

Basically, if 9 GeV to 10 TeV mass dark matter particles exist, they have to have be completely "sterile", i.e. have no non-gravitational interactions with ordinary matter.

See also Zachary Bogorad, Peter Graham, Harikrishnan Ramani, "Constraints on Long-Ranged Interactions Between Dark Matter and the Standard Model", arXiv:2410.07324 (October 9, 2024) (tightly constraining possible ordinary matter-dark matter interactions by a completely independent method).

Of course, this poses its own problem.

Dark matter distributions that are inferred from galaxy dynamics and lensing data are far more predictable from observable ordinary matter distributions (pretty much completely to the limit of measurement error, see Federico Lelli, Stacy S. McGaugh, and James M. Schombert, "The small scatter of the baryonic Tully-Fisher relation" (December 14, 2015)), which shouldn't be possible if dark matter particles have no non-gravitational interactions with ordinary matter. See, e.g., Paolo Salucci and Nicola Turini, "Evidences for Collisional Dark Matter In Galaxies?" (July 4, 2017) and Paolo Salucci, "The distribution of dark matter in galaxies" (November 21, 2018) (60 pages, 28 Figures ~220 refs. Invited review for The Astronomy and Astrophysics Review) and Edo van Uitert, et al., "Halo ellipticity of GAMA galaxy groups from KiDS weak lensing" (October 13, 2016).

And, inferred dark matter halos for the most part, don't have the NFW halo shape that truly sterile dark matter candidates that are too massive to have significant wave-like behavior (or self-interactions that don't involve ordinary matter) should have. See, e.g., Jorge Sanchez Almeida, "Einasto gravitational potentials have difficulty to hold spherically symmetric stellar systems with cores" arXiv:2406.13613 (June 19, 2024) (RNAAS complementing our previous paper Sanchez Almeida et al. (2023, ApJ, 954, 153; doi: https://doi.org/10.3847/1538-4357/ace534)) and Jorge Sanchez Almeida, Angel R. Plastino, Ignacio Trujillo, "Can cuspy dark matter dominated halos hold cored stellar mass distributions?" arXiv:2307.01256 (July 3, 2023) (Accepted for publication in ApJ).

Furthermore, no one has convincingly explained these discrepancies with physically plausible baryonic feedback effects, which are often suggested as a possible explanation for what is observed. Indeed, a 2023 paper recounts a galaxy known as Nube, which should not be possible if the necessary baryonic feedback to address the small scale problems of sterile dark matter exists.

Self-interacting dark matter (SIDM) also has tight constraints, see, e.g., Shin'ichiro Ando, et al., "Stringent Constraints on Self-Interacting Dark Matter Using Milky-Way Satellite Galaxies Kinematics" arXiv:2503.13650 (March 17, 2025), and is realistically over constrained with some constraints ruling out all of the parameter space allowed by other constraints. See, e.g., Ziwen Zhang, et al., "Unexpected clustering pattern in dwarf galaxies challenges formation models" arXiv:2504.03305 (April 7, 2025) (Accepted for publication in Nature) (favoring of cross-section of interaction of 3.0 rather than under 0.2, in the same units). See also here (cross-section of more than 2.0 favored, published at 452 MNRAS 1468 (2015)).

Really, the only dark matter particle candidates that are not really strongly challenged by the observational evidence are those with extremely light dark matter particles (e.g., in fuzzy dark matter models and axion like particle (ALP) models), with masses much lighter than the average neutrino mass (ca. 10^-20 eV to 10^-24 eV), that can't arise from a thermal freeze-out model and have the low mean velocities inferred from the amount of large scale structure and galaxy structure that is observed. See generally, Tonatiuh Matos, Luis A. Ureña-López, Jae-Weon Lee, "Short Review of the main achievements of the Scalar Field, Fuzzy, Ultralight, Wave, BEC Dark Matter model" arXiv:2312.00254 (November 30, 2023). These hypothetical bosons, coincidentally, have masses of the same rough order of magnitude as the mass-energy of a hypothetical typical graviton.

This isn't to say that these ultra-light dark matter candidates are correct, however. This dark matter particle candidate is a relatively new one to receive serious attention, and scientists haven't fully "kicked the tires" yet on these models as rigorously as they have for older dark matter particle candidates.

See also, this previous thread at PF addressing a very similar question.

 

[Orodruin]

Neutrinos have standard strength weak force interactions, and we know from neutrino telescope observations that there aren't nearly enough of them in the universe to account for the inferred total mass of dark matter particles).

Neutrino telescopes only detect very (or ultra) high energy neutrinos, typically originating from cosmogenic sources such as AGNs. The main problem with neutrinos is that they are too light - both to make up a significant portion of the matter budget and leading to them being a warm dark matter candidate in tension with observed structures.

 

[snorkack]

Assuming a dark matter particle paradigm, according to a pre-print by Yang (2015) subsequently published in Physical Review D, the lower bound on the mean lifetime of dark matter particles is 3.57×10²⁴ seconds. This is roughly 10¹⁷ years. By comparison the age of the universe is roughly 1.38×10¹⁰ years.

This means that dark matter (if it exists) is at least as stable as anything other than a proton, which has an experimentally determined mean lifetime of at least 10³⁴ years, or an electron, which is theoretically stable (just as the proton is in the Standard Model) and has an experimentally determined mean lifetime of at least 6.6×10²⁸ years.

Turns out that the experimental decay mode against which electron stability was checked against was a charge nonconserving mode
e^-=ν+γ
And that´s possible at halflife of 10²⁹ years? Note how it charges up the world! (positrons are rare and protons are "proven" to have 300 000 times longer halflife!)
How was it searched? Unexplained disappearance of negative charge? Unexplained appearance of a γ?
In case of charge nonconservation, to what halflife can e^-=2ν, e^-=3ν or e^-=2Wimp be ruled out?
About the proton: these 10³⁴ year halflives refer to searches for Cherenkov emitting charged energetic decay products e⁺ and μ⁺.
How about processes like p+e^-=2ν or p+e^-=2Wimp? To what halflives are unexplained disappearances of protons that do NOT produce energetic standard model particles observationally limited?

Why are there so much searches about "dark matter annihilation" producing standard model particles? Do we have any a priori reason to suppose that standard model particles are the preferred and the more stable form of matter? Could it be the case that spontaneous interconversion, slow as it is, goes from standard model matter towards dark matter because that´s the ultimately stable form of matter?

About dark matter particles being "stable" or resistant to "annihilation" - yes, sure, we would see their decay or annihilation products if they were standard model particles except maybe neutrinos.
But could some dark matter particles be unstable to decay or annihilation, on astronomically relevant timescales, if it is to different dark matter particles?
Say processes like
cWimp=2hWimp
or
2cWimp=2hWimp
Would spontaneous and irreversible conversion of cold dark matter into hot dark matter by decay (proportional to cWimp density) or annihilation (proportional to square of cWimp density) leave observational evidence? To what extent is it ruled out? Because if dark matter decays to other forms of dark matter, it is not "stable".

 

[Orodruin]

In case of charge nonconservation, to what halflife can e-=2ν, e-=3ν or e-=2Wimp be ruled out?

$e\to \nu\nu$ and $e\to \chi\chi$ would violate conservation of angular momentum. I’m not saying it can’t happen, but it would shake the very foundations of physics - even more than just electron decay.

Do we have any a priori reason to suppose that standard model particles are the preferred and the more stable form of matter?

Generally, WIMPs are heavier than SM particles. In particular heavier than electrons and neutrinos. It is a matter of kinematics.

we would see their decay or annihilation products if they were standard model particles except maybe neutrinos.

Neutrinos from dark matter annihilation is a perfectly viable search mode.

 

[snorkack]

$e\to \nu\nu$ and $e\to \chi\chi$ would violate conservation of angular momentum. I’m not saying it can’t happen, but it would shake the very foundations of physics - even more than just electron decay.

It is interesting that the longhaul consequences of Coulomb repulsion are supposed to be easier to handle than the longhaul consequences of angular momentum!
Earth has angular momentum of 7*10³³ kg/m²/s due to rotation, which makes Earth spin about 7*10⁶⁷. I do not know whether Earth is at the moment a fermion or a boson but I suspect it varies often as half-spin particles, atoms and molecules arrive and depart Earth.
The capacitance of Earth is a mere 710 μF.
If 1 C of electrons - about 6*10¹⁸ of them - were to decay without conserving their charge (and leaving the protons in place), that would charge the Earth by 1400 V. Would we notice?
If 1 C of electrons were to decay without conserving their spin, the Earth day would change by, oh, 3*10¹⁸ divided by 7*10⁶⁷ would give a fraction of 2*10⁴⁹!
Earth mass is 6*10²⁷ g, and this includes about 3.6*10⁵¹ nucleons, slightly under half of which are protons. The combined charge of all protons in Earth is a bit under 6*10³² C. And the combined spin of all fundamental particles Earth consists of, protons, neutrons and electrons combined, is about 2.5*10⁵¹. The rest of the Earth spin of about 7*10⁶⁷ is orbital angular momentum of Earth rotation.

 

[ohwilleke]

Neutrino telescopes only detect very (or ultra) high energy neutrinos, typically originating from cosmogenic sources such as AGNs. The main problem with neutrinos is that they are too light - both to make up a significant portion of the matter budget and leading to them being a warm dark matter candidate in tension with observed structures.

The core point is that we can make up a matter budget for them and that there aren't enough of them by many, many orders of magnitude.

Neutrinos would also be a "hot dark matter" candidate and is indeed in tension with the amount of structure observed at scales like those of galaxies, galaxy clusters, and the "cosmic web". Qualitatively, hot dark matter means that the dark matter particles have mean velocities so high that they prevent those kinds of structures from arising. Neutrinos are generally relativistic or nearly so, because even the least bit of energy is enough to propel their tiny masses to extreme velocities.

The current upper bound on the absolute mass of the lightest neutrino is 0.45 eV from KATRIN direct measurement (implying as sum of the three neutrino masses of not more than 1.45 eV or so). KATRIN is expected to reduce that upper bound on the lightest neutrino mass to 0.2 eV at the completion of its run in the next few years (implying as sum of the three neutrino masses of not more than 0.7 eV or so). The minimum sum of the three neutrino masses if the lightest neutrino mass is basically zero is about 0.06 eV in a normal hierarchy of neutrino masses (which is mildly favored by observations) and about 0.1 eV in an inverted hierarchy of neutrino masses). Cosmology based bounds on the sum of the three neutrino masses are on the order of 0.12 eV but converting astronomy observations to neutrino masses is model dependent with DESI showing a mild statistical preference for a very low bound below the floor established in neutrino oscillation experiments. Still, if the cosmology bounds are anywhere close to right, and they are in synch with the scale of neutrino masses from neutrino oscillation experiments, the lightest neutrino mass in on the order of 0.001 to 0.020 meV or less.

Warm Dark Matter Compared

The term "warm dark matter" is generally reserved for sterile or nearly sterile dark matter candidates with masses on the order of 1 keV if it has thermal freeze-out origins, which is about 100,000 to 1,000,000+ times more massive than neutrinos. Qualitatively, warm dark matter means that the dark matter particles have essentially the same properties as the paradigmatic "cold dark matter" (basically anything with a mass >> keV in a thermal freeze out scenario), but the critical point about it is not its mean velocity (although it is higher than "cold dark matter") but it particle mass which impacts its Compton wave-length, which is just small enough for it to start displaying meaningful wave-like quantum behavior at wave-lengths long enough to have an observable impact on galaxy dynamics and astronomy observations.

Dark matter phenomena are observed to have some wave-like behavior. See Alfred Amruth, "Einstein rings modulated by wavelike dark matter from anomalies in gravitationally lensed images" Nature Astronomy (April 20, 2023) https://doi.org/10.1038/s41550-023-01943-9 (Open access copy available at arxiv).

Warm dark matter's wave-like behavior is due to quantum physics considerations helps address some of the small scale structure problems with cold dark matter, especially the "core-cusp" problem (identified, e.g., in Boylan-Kolchin et al. 2011), and the failure to real world inferred dark matter halo shapes to match the NFW profile (noted in #6; (see also, e.g., James S. Bullock, Michael Boylan-Kolchin, "Small-Scale Challenges to the ΛCDM Paradigm" (July 13, 2017, last updated September 2, 2019); Nicolas Loizeau, Glennys R. Farrar, “Galaxy rotation curves disfavor traditional and self-interacting dark matter halos, preferring a disk component or ad-hoc Einasto function” arXiv 2105:00119 (April 30, 2021); Daniel B Thomas, Michael Kopp, Katarina Markovič, "Using large scale structure data and a halo model to constrain Generalised Dark Matter" arXiv:1905.02739 (May 7, 2019 last updated May 4, 2020) https://doi.org/10.1093/mnras/stz2559), and Theodorus Maria Nieuwenhuizen "Subjecting dark matter candidates to the cluster test" (October 3, 2017)).

Warm dark matter was a very promising prospect a couple of decades or so ago when it was first proposed in a well-thought out manner. See e.g., Sommer-Larsen & Dolgov 2001; Bode et al. 2001). See also Alyson Brooks, "Re-Examining Astrophysical Constraints on the Dark Matter Model" (July 28, 2014) (identifying WDM and SIDM as more promising the CDM which was almost ruled out observationally by then).

But further analysis determined that it is too mild a cure for cold dark matter's small scale structure problems, and other observations have also constrained it.

One common version of warm dark matter, for example, is keV scale sterile neutrino dark matter. But observations strongly disfavor it at scales below about 4 keV, which is pushing the high end of the mass range where its quantum features can be shown. See Oliver Newton, et al., "Constraints on the properties of νMSM dark matter using the satellite galaxies of the Milky Way" arXiv:2408.16042 (August 28, 2024), Simon Birrer, Adam Amara, and Alexandre Refregier, "Lensing substructure quantification in RXJ1131-1231: A 2 keV lower bound on dark matter thermal relict mass" (January 31, 2017), Schneider (2017), and Viel (2013).

Early observational hints of warm dark matter annihilation or decay have now been largely ruled out as having that cause. See Christopher Dessert, Joshua W. Foster, Yujin Park, Benjamin R. Safdi, "Was There a 3.5 keV Line?" arXiv:2309.03254 (September 6, 2023). See also https://arxiv.org/abs/1408.1699and https://arxiv.org/abs/1408.4115.

Warm dark matter shares the same problems as sterile cold dark matter in terms of the lack of a mechanism causing it to be so tightly linked to ordinary matter distributions, noted in Paolo Salucci and Nicola Turini, "Evidences for Collisional Dark Matter In Galaxies?" (July 4, 2017) and, e.g. Camila A. Correa, Joop Schaye, "The dependence of the galaxy stellar-to-halo mass relation on galaxy morphology" arXiv:2010.01186 (October 2, 2020) (accepted for publication in MNRAS), and also can't explain observations such as the observation that:

[A]t equal luminosity, flattened medium-size elliptical galaxies are on average five times heavier than rounder ones, and . . . the non-baryonic matter content of medium-size round galaxies is small.

A. Deur, "A correlation between the dark content of elliptical galaxies and their ellipticity" arXiv:2010.06692 (October 13, 2020) (the paper details the analysis of the results published in MNRAS 438, 2, 1535 (2014) reporting an empirical correlation between the ellipticity of elliptical galaxies and their dark matter content).

Another problem with warm dark matter is that is delays galaxy formation by ca. 200 million years relative to LambdaCDM, when observation suggests that galaxy formation happens sooner than LambdaCDM assumptions would support. See Lovell (2020).

 

[ohwilleke]

Turns out that the experimental decay mode against which electron stability was checked against was a charge nonconserving mode
e^-=ν+γ
And that´s possible at halflife of 10²⁹ years? Note how it charges up the world! (positrons are rare and protons are "proven" to have 300 000 times longer halflife!)

The upper bounds are merely a result of the limits of experimental precision. These upper bounds don't imply any positive evidence that it is possible for a longer time period. The experiments simply aren't precise enough and have large enough sample sizes to rule out longer half-lives yet.

Neither electron decay nor proton decay are allowed in the Standard Model because there aren't, as you observe, possible decay products in the form of Standard Model particles that conserve the quantum numbers and mass-energy conservation that apply in the Standard Model. These Standard Model conservation laws include electromagnetic charge conservation (as you note in the case of electron decay), and, in the case of proton decay, multiple reasons at the energies found in the current cosmological era, including baryon number conservation which is preserved in the Standard Model (absent ultra-high energy sphaleron interactions that would take colliders with about 100 times the energy of the LHC to produce, energies which have not been present in nature since very shortly after the Big Bang).

Proton decay is a subject of experimental interest because many grand unification theories (GUTs) that try to unify the Standard Model interactions into a single Lie group, unlike the Standard Model which imposes this conservation law, do not mandate baryon number conservation above moderately high energies that have already been explored experimentally. The non-detection of proton decay to high precision has experimentally ruled out the simplest GUT models such as SU(5) unification of the Standard Model forces. Increased precision in ruling out proton decay can rule out more complicated GUT models at even higher energies that can be reached directly in high energy collider experiments.

But, the bottom line is that not even the slightest hint of electron decay or proton decay has been observed in any kind of experimental search, and that these decays are not allowed or expected in the Standard Model.

And, this, of course, is relevant at all in this discussion simply to illustrate that dark matter particles, if they exist, must be among the most stable particles in the universe. This is exceptional because the vast majority of fundamental and composite particles allowed by the Standard Model have mean lifetimes on the order of a microsecond to time frames eighteen orders of magnitude shorter than that. There are no stable massive bosons in the Standard Model. The only fermions in the the Standard Model that are stable (all of which are massive) are two kinds of baryons (protons and bound neutrons in stable atoms), electrons, and neutrinos (which themselves oscillate).

Why are there so much searches about "dark matter annihilation" producing standard model particles? Do we have any a priori reason to suppose that standard model particles are the preferred and the more stable form of matter? Could it be the case that spontaneous interconversion, slow as it is, goes from standard model matter towards dark matter because that's the ultimately stable form of matter?

About dark matter particles being "stable" or resistant to "annihilation" - yes, sure, we would see their decay or annihilation products if they were standard model particles except maybe neutrinos.
But could some dark matter particles be unstable to decay or annihilation, on astronomically relevant timescales, if it is to different dark matter particles?

This comes down, in part, to the parable about searching for your lost keys under the street light, even if they are more likely to be in the dark part of the alley, where they simply can't be found until sunrise.

We search for decays into Standard Model particles because if they exist, we can find them, and if they don't, we have a new property that dark matter must obey, which is that it doesn't have interactions that annihilate or decay into Standard Model particles (i.e. it is "sterile" in the sense of not having any interactions with any of the three fundamental forces of the Standard Model, even if there is a new physics dark matter to dark matter force).

The more evidence we have the dark matter is sterile (in this sense), the more puzzling it is that gravitational interactions with ordinary matter alone can cause inferred dark matter halos to be so tightly linked to ordinary matter distributions in galaxies.

A perfectly sterile dark matter sector also favors thermal freeze-out models to alternatives where dark matter is constantly being created and destroyed in an equilibrium manner.

This is because some sort of process has to create it, and if it is that sterile, the easiest way for that to be the case would be an interaction that only takes place in the terra incognita of the ultra-high energies shortly after the Big Bang, where we can't rule out new physics because we can't create experiments that reach such high energies.

New physics outside the experimentally confirmed domain of applicability of the Standard Model are a lot more palatable than new physics that can create and destroy dark matter particles at energy scales (such as those present in deep outer space where dark matter is inferred to be and has been for the last 13 billion plus years) that have been exhaustively studied in laboratories.

Of course, if dark matter particles decay into other dark matter particles that have the same observable consequences in astronomy, to some extent, we don't really care. All possible dark matter particles with the same observable consequences in astronomy are functionally the same things for purposes of astrophysics, which is the discipline of physics that motivates the search for dark matter and places bounds on its properties if it exists, in the first place. If multiple different kinds of dark matter particles all have the same observable consequences then the exact properties of dark matter particles are, to that extent, theoretically unknowable.

Dark matter annihilation or decay into other dark matter particles, however, in most scenarios, would have observable consequences. It might change mean dark matter particle velocity (i.e. making the average composition of dark matter hotter or cooler over time). It would influence the momentum of dark matter halos over time. It might influence the migration of visible stars inward or outward within galaxies. And, so on.

In general, dark matter annihilation or decay into other dark matter particles on time scales significantly less than the age of the universe (e.g. a half-life of several billion years or less) would imply that the dynamics of galaxies and galaxy clusters in the early universe (e.g., in galaxies and clusters that only the Hubble telescope and the JWST are powerful enough to see) should be materially different from galaxies and galaxy clusters at intermediate and low redshifts, respectively.

So far, however, the recent observations of the JWST are telling us the opposite story. The earliest galaxies whose dynamics we can discern (which aren't quite as early as the earliest galaxies that can be seen at all) look very similar to modern galaxies (except for their lower average metallicity, the cause of which is well understood).

 

[snorkack]

The upper bounds are merely a result of the limits of experimental precision. These upper bounds don't imply any positive evidence that it is possible for a longer time period. The experiments simply aren't precise enough and have large enough sample sizes to rule out longer half-lives yet.

The experiments for electron lifetime have sample size of the whole world. Locally charge nonconserving electron decay would have no reason to be balanced by positron decay (positrons are rare) or proton decay (protons are different) and buildup of charge imbalance, even relatively tiny amounts, would cause a long distance electrostatic field which would tend to be detected.

 

[Orodruin]

The core point is that we can make up a matter budget for them and that there aren't enough of them by many, many orders of magnitude.

Neutrinos would also be a "hot dark matter" candidate and is indeed in tension with the amount of structure observed at scales like those of galaxies, galaxy clusters, and the "cosmic web". Qualitatively, hot dark matter means that the dark matter particles have mean velocities so high that they prevent those kinds of structures from arising. Neutrinos are generally relativistic or nearly so, because even the least bit of energy is enough to propel their tiny masses to extreme velocities.

The current upper bound on the absolute mass of the lightest neutrino is 0.45 eV from KATRIN direct measurement (implying as sum of the three neutrino masses of not more than 1.45 eV or so). KATRIN is expected to reduce that upper bound on the lightest neutrino mass to 0.2 eV at the completion of its run in the next few years (implying as sum of the three neutrino masses of not more than 0.7 eV or so). The minimum sum of the three neutrino masses if the lightest neutrino mass is basically zero is about 0.06 eV in a normal hierarchy of neutrino masses (which is mildly favored by observations) and about 0.1 eV in an inverted hierarchy of neutrino masses). Cosmology based bounds on the sum of the three neutrino masses are on the order of 0.12 eV but converting astronomy observations to neutrino masses is model dependent with DESI showing a mild statistical preference for a very low bound below the floor established in neutrino oscillation experiments. Still, if the cosmology bounds are anywhere close to right, and they are in synch with the scale of neutrino masses from neutrino oscillation experiments, the lightest neutrino mass in on the order of 0.001 to 0.020 meV or less.

Warm Dark Matter Compared

The term "warm dark matter" is generally reserved for sterile or nearly sterile dark matter candidates with masses on the order of 1 keV if it has thermal freeze-out origins, which is about 100,000 to 1,000,000+ times more massive than neutrinos. Qualitatively, warm dark matter means that the dark matter particles have essentially the same properties as the paradigmatic "cold dark matter" (basically anything with a mass >> keV in a thermal freeze out scenario), but the critical point about it is not its mean velocity (although it is higher than "cold dark matter") but it particle mass which impacts its Compton wave-length, which is just small enough for it to start displaying meaningful wave-like quantum behavior at wave-lengths long enough to have an observable impact on galaxy dynamics and astronomy observations.

Dark matter phenomena are observed to have some wave-like behavior. See Alfred Amruth, "Einstein rings modulated by wavelike dark matter from anomalies in gravitationally lensed images" Nature Astronomy (April 20, 2023) https://doi.org/10.1038/s41550-023-01943-9 (Open access copy available at arxiv).

Warm dark matter's wave-like behavior is due to quantum physics considerations helps address some of the small scale structure problems with cold dark matter, especially the "core-cusp" problem (identified, e.g., in Boylan-Kolchin et al. 2011), and the failure to real world inferred dark matter halo shapes to match the NFW profile (noted in #6; (see also, e.g., James S. Bullock, Michael Boylan-Kolchin, "Small-Scale Challenges to the ΛCDM Paradigm" (July 13, 2017, last updated September 2, 2019); Nicolas Loizeau, Glennys R. Farrar, “Galaxy rotation curves disfavor traditional and self-interacting dark matter halos, preferring a disk component or ad-hoc Einasto function” arXiv 2105:00119 (April 30, 2021); Daniel B Thomas, Michael Kopp, Katarina Markovič, "Using large scale structure data and a halo model to constrain Generalised Dark Matter" arXiv:1905.02739 (May 7, 2019 last updated May 4, 2020) https://doi.org/10.1093/mnras/stz2559), and Theodorus Maria Nieuwenhuizen "Subjecting dark matter candidates to the cluster test" (October 3, 2017)).

Warm dark matter was a very promising prospect a couple of decades or so ago when it was first proposed in a well-thought out manner. See e.g., Sommer-Larsen & Dolgov 2001; Bode et al. 2001). See also Alyson Brooks, "Re-Examining Astrophysical Constraints on the Dark Matter Model" (July 28, 2014) (identifying WDM and SIDM as more promising the CDM which was almost ruled out observationally by then).

But further analysis determined that it is too mild a cure for cold dark matter's small scale structure problems, and other observations have also constrained it.

One common version of warm dark matter, for example, is keV scale sterile neutrino dark matter. But observations strongly disfavor it at scales below about 4 keV, which is pushing the high end of the mass range where its quantum features can be shown. See Oliver Newton, et al., "Constraints on the properties of νMSM dark matter using the satellite galaxies of the Milky Way" arXiv:2408.16042 (August 28, 2024), Simon Birrer, Adam Amara, and Alexandre Refregier, "Lensing substructure quantification in RXJ1131-1231: A 2 keV lower bound on dark matter thermal relict mass" (January 31, 2017), Schneider (2017), and Viel (2013).

Early observational hints of warm dark matter annihilation or decay have now been largely ruled out as having that cause. See Christopher Dessert, Joshua W. Foster, Yujin Park, Benjamin R. Safdi, "Was There a 3.5 keV Line?" arXiv:2309.03254 (September 6, 2023). See also https://arxiv.org/abs/1408.1699and https://arxiv.org/abs/1408.4115.

Warm dark matter shares the same problems as sterile cold dark matter in terms of the lack of a mechanism causing it to be so tightly linked to ordinary matter distributions, noted in Paolo Salucci and Nicola Turini, "Evidences for Collisional Dark Matter In Galaxies?" (July 4, 2017) and, e.g. Camila A. Correa, Joop Schaye, "The dependence of the galaxy stellar-to-halo mass relation on galaxy morphology" arXiv:2010.01186 (October 2, 2020) (accepted for publication in MNRAS), and also can't explain observations such as the observation that:



A. Deur, "A correlation between the dark content of elliptical galaxies and their ellipticity" arXiv:2010.06692 (October 13, 2020) (the paper details the analysis of the results published in MNRAS 438, 2, 1535 (2014) reporting an empirical correlation between the ellipticity of elliptical galaxies and their dark matter content).

Another problem with warm dark matter is that is delays galaxy formation by ca. 200 million years relative to LambdaCDM, when observation suggests that galaxy formation happens sooner than LambdaCDM assumptions would support. See Lovell (2020).

Uhmm … yea, obviously ”warm” was a slip of tongue and nothing else. There is no need to write a 20 page essay about it …

 

[ohwilleke]

Uhmm … yea, obviously ”warm” was a slip of tongue and nothing else. There is no need to write a 20 page essay about it …

In my world, being thorough is everything and one missed detail can be catastrophic. It's an ingrained habit.

 

[Orodruin]

In my world, being thorough is everything and one missed detail can be catastrophic. It's an ingrained habit.

I mean, writing an essay about something you know I know just seems like a waste of time. ”Did you mean ’hot’?” would have taken 5 seconds to write.

 

[pinball1970]

I mean, writing an essay about something you know I know just seems like a waste of time. ”Did you mean ’hot’?” would have taken 5 seconds to write.

There are unqualified observers though. We you know that you know this stuff, but "we" don't.
By we I mean me of course.

 

[Jaime Rudas]

The LambdaCDM "Standard Model of Cosmology" assumes a constant amount of dark matter in the universe after the earliest moments of the universe (with the density of the dark matter in the universe decreasing in proportion to the spatial volume of the universe), just as the model does in the case of ordinary baryonic matter.

Shouldn't it be "with the density of the dark matter in the universe decreasing in proportion to the increase in the spatial volume of the universe" or, better, "with the density of the dark matter in the universe decreasing in proportion to the increase in the expansion of the universe"?

 

[WWGD]

Shouldn't it be "with the density of the dark matter in the universe decreasing in proportion to the increase in the spatial volume of the universe" or, better, "with the density of the dark matter in the universe decreasing in proportion to the increase in the expansion of the universe"?

Is this ratio approaching 0? If so, how fast?

 

[Jaime Rudas]

Is this ratio approaching 0?

Yes, in the sense that any positive value that decreases approaches zero.

If so, how fast?

At the same rate at which the volume of space expands.

 

[ohwilleke]

Shouldn't it be "with the density of the dark matter in the universe decreasing in proportion to the increase in the spatial volume of the universe" or, better, "with the density of the dark matter in the universe decreasing in proportion to the increase in the expansion of the universe"?

Sure. I was a little sloppy in how I worded that.