# What is Dark Matter or Energy?

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• Zeal Faust

#### Zeal Faust

I have read that almost 80 to 90% of our space is made of dark matter or Dark Energy. But what is the function of this Dark matter or energy? Is this something that binds stars, galaxies, etc? If not then what is the actual matter with Dark energy?

We don't know. Dark matter is, apparently, only observable via its gravitational interactions, not by light, making it very difficult to understand. We can't put it in a test tube in a lab and poke it with an electrode, or send a probe out into space to bring some back. We only observe that galaxies don't have the right rotation rates and that sometimes the amount of matter we can see doesn't match up with how much gravity there appears to be in an area. Our best explanation at the moment is dark matter.

Dark energy is even more elusive. It's not matter, it's not radiation, and like dark matter we can't just reach out and grab some to poke and prod it. It's something that apparently may be causing the universe to expand differently than it would otherwise. How it does so is very difficult to explain if you don't know about general relativity, metrics, tensors, and some other advanced mathematical things. And I don't know much about any of those, so I couldn't begin to give you a good explanation.

vanhees71 and ohwilleke
We don't know. Dark matter is, apparently, only observable via its gravitational interactions, not by light, making it very difficult to understand. We can't put it in a test tube in a lab and poke it with an electrode, or send a probe out into space to bring some back. We only observe that galaxies don't have the right rotation rates and that sometimes the amount of matter we can see doesn't match up with how much gravity there appears to be in an area. Our best explanation at the moment is dark matter.

Dark energy is even more elusive. It's not matter, it's not radiation, and like dark matter we can't just reach out and grab some to poke and prod it. It's something that apparently may be causing the universe to expand differently than it would otherwise. How it does so is very difficult to explain if you don't know about general relativity, metrics, tensors, and some other advanced mathematical things. And I don't know much about any of those, so I couldn't begin to give you a good explanation.
So In simple, we know almost nothing about dark matter but we know that it exists based on observations?

If that is so, What if they are nothing like how we are thinking? I mean according to your words that "It's something that apparently may be causing the universe to expand differently than it would otherwise" what if we think from another point, That there is something extremely powerful with a much larger force pulling our Universe to Expand?

Not that I don't understand your point, In fact, your words are very clear, My confusion is we know nothing about the dark matter more so dark energy yet we say Universe is almost 90% Dark Matter and energy.

What if they are nothing like how we are thinking?
Welcome to PF.

Welcome to PF.

I am sorry sir! I will be careful

berkeman
Generally, if you ask "what is dark matter" we'll tell you what dark matter is and not mention alternatives. Don't assume that means that there are no alternatives under consideration. That said, at this time dark matter and dark energy are our best explanations for certain phenomena.

Alternative gravity theories have been proposed to explain dark matter, but they can't explain the large scale structure of the universe where conventional gravity with dark matter does. So dark matter is still the preferred theory at the moment, but alternatives are under consideration.

Dark energy is a different phenomenon. I understand that "we're in an underdense region" (which is a more plausible model than "something is pulling outwards", but has much the same effect) has been considered but rejected because it can't be made to fit the data. So, again, dark energy remains the preferred hypothesis at the moment.

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martinbn, vanhees71, PeroK and 1 other person
If that is so, What if they are nothing like how we are thinking?
No problem. We'll alter our theories as more data comes in. That's exactly how science works.
I mean according to your words that "It's something that apparently may be causing the universe to expand differently than it would otherwise" what if we think from another point, That there is something extremely powerful with a much larger force pulling our Universe to Expand?
If that's the case then this force pulling the universe apart is the 'something' causing it to expand differently than it otherwise would, so that's not really a different point of view, just a guess at what might be causing it.

vanhees71, Zeal Faust and Ibix
@Zeal Faust It's important to understand that Dark Matter (DM) and Dark Energy (DE) are two very different things, and they exist to explain two very different sets of observed phenomena. Dark Matter exists to explain apparent "missing mass". I.e. Given what General Relativity (GR) says about how gravity works, physical systems on a wide variety of physical scales appear not to have enough mass to hold themselves together, given the amount of luminous matter we can observe within them.

Whereas, Dark Energy exists to explain the observation that the expansion of the Universe is accelerating, which is contrary to the predictions of GR for a Universe containing only matter (including DM!) and radiation (photons).

I would say that current observational & experimental data contain quite a bit of (admittedly indirect) evidence for the existence of Dark Matter. EDIT: E.g. we can basically map out what its distribution is in space (in some places) using Gravitational Lensing measurements. I'd also argue that the data we've collected have placed quite a few more constraints on the nature of Dark Matter than on Dark Energy. E.g.
• We know that Dark Matter must be made up of some new type of particle that is not one of the ones from the Standard Model of Particle Physics
• We know that this particle must have mass. I think (depending on the specific theory/type of Dark Matter being considered) we are even able to constrain the DM particle mass to be within a certain range (error bar). I don't know what that is, off the top of my head
• We know that the DM particle must be NON-baryonic in nature, due to cosmological models of structure formation, which show that the baryons alone are insufficient to produce the level of inhomogeneity (non-smoothness) to the matter we see in the Universe at the present day, given the starting conditions (from the CMB)
• We know that the DM particle must be capable of interacting with other matter only through the Gravitational force and (maybe) the Weak Nuclear Force. But not the other two fundamental forces of nature (Electromagnetic Force & Strong Nuclear Force). In particular, if DM could interact via the electromagnetic force, it wouldn't be invisible!
• As a corollary to the above, we know that the DM particle has no electric charge.
Dark Matter "Direct Detection" laboratory experiments are a thing. E.g. under the assumption that the DM particle is capable of Weakly interacting (like neutrinos) and just does so very rarely, detectors can be set up underground that try to increase the cross section for such interactions. We haven't found anything yet. But the point I'm trying to make with the above is that just because we haven't found the particle does not mean that the data we've collected (both astronomical & direct detection) have afforded us zero information about its necessary properties.

Dark Energy, on the other hand, is just some postulated mystery "scalar field" that may permeate all of space. If it's there, it could readily explain the observed acceleration of the Universe's expansion. But we know almost nothing about its nature. The (energy) density of this field may be constant (corresponding to a Cosmological Constant term in the Friedmann equations). Or the energy density of DE may have some slow evolution with time. We don't have enough data to be sure, but future galaxy-survey telescopes like Euclid & WFIRST that will look at the evolution of the matter distribution w/ redshift on large scales in more detail than before, might be able to tell us more about that.

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TeethWhitener, ohwilleke, vanhees71 and 3 others
Dark matter and dark energy are hypothetical mechanisms to explain to different sets of phenomena observed in astronomers. Scientists have thus far been unable to reach a consensus explanation for these phenomena with the Standard Model of Particle Physics taken together with General Relativity without a cosmological constant. These mechanisms would apply "new physics" to explain what we see.

Dark Matter

Dark matter phenomena involve the fact that the stars and interstellar matter that we can observe moves and clumps and generates cosmic microwave background radiation to form patterns in ways that it shouldn't be able to if only Newtonian gravity (which is used as a close approximation of weak field general relativity in astronomy and cosmology applications) was present.

Roughly speaking, in many circumstances where gravitational fields from ordinary matter are very weak, ordinary matter seems to be more tightly bound to other ordinary matter than we would expect it to be. Also, the observations at the cosmology level assume that there has been a fixed amount of dark matter in the Universe from the time that the cosmic background radiation patterns were established, if not earlier.

We don't know what causes dark matter phenomena and haven't come up with any proposals that adequately explain all the data.

The paradigm is "cold dark matter". But that hypothesis, in its original simple form, has so many independent observations of different kinds contradicting it, that the original simple explanation (collisionless particles that interact only via gravity with a mass on the order of 1-1000 GeV/c^2 if modeled in a thermal relic freeze out model of dark matter formation) cannot possibly be correct. There are close to a dozen serious discrepancies depending upon how you count them, some of which related to galaxy scale dynamics, some of which arise at the cluster scale, and some of which involve cosmology level problems.

We have also ruled out lots and lots and lots of different hypotheses, other than the simplest versions of dark matter particles, with observational evidence. But there are plenty of dark matter particle hypotheses that continue to be investigated and have not been ruled out with observational evidence, although for most of these hypotheses we have narrowed down the possible properties of these dark matter particles (sometimes more than one kind) considerably.

Some of the leading theories right now are warm dark matter (WDM) (with masses on the order of keV/c^2 per particle), sterile right handed neutrinos, axion-like particles (ALP) (that have very, very low masses), primordial black holes (PBHs) formed in the early Universe with masses on the order of typical asteroids, and self-interacting dark matter (SIDM) (with a fifth forth mediator particle that creates a force between dark matter particles).

We have also considered whether dark matter phenomena can be explained by modifications to General Relativity as conventionally applied in astronomy and cosmology circumstances. Some of these proposals are also not a good fit to all of the data, others haven't received enough attention to be proven or disproven. The most well know of these proposals is called MOND (for MOdified Newtonian Dynamics) a theory proposed in 1983 which does an excellent job of explaining observations at the galaxy scale and below with just one additional physical constant, even in its simplest toy model form, but underestimates dark matter phenomena at galaxy cluster scales and larger structures and doesn't have an obvious relativistic generalization. One attempt at a relativistic generalization of it, however, has been fit to the cosmic background radiation patterns observed, and it also does better than the cold dark matter particle hypothesis at explaining why galaxies form earlier than expected and why 21cm wave length backgrounds look the way that they do.

One of the challenges for dark matter particle theories is explaining why a simple tweak to the law of gravity, while it doesn't replicate all of the dark matter phenomena observed, does reproduce so much of it (although not all observations) and does such a good job of predicting new observations in untested situations, when a simple dark matter particle theory wouldn't obviously do so.

This challenge also illustrates another key point. Even if we knew that dark matter phenomena were caused by one or more particles with a particular set of properties, that isn't really a complete explanation of dark matter phenomena. We also need to know why, in that hypothesis, dark matter is distributed within the Universe in the manner in which it is, and in particular, why dark matter distributions in galaxies are so tightly correlated with distributions of ordinary matter in galaxies. But, in dark matter particle theories, this requires getting into the weeds of very complex galaxy assembly processes that we don't fully understand.

Dark Energy

Dark energy is a concept motivated pretty much entirely by the observation that the expansion of the Universe should happen more slowly than it is observed to, and should not appear to accelerate in the way that it appears to, in the absence of something with an effect similar to the cosmological constant of General Relativity having a very small value.

In its simplest form, a cosmological constant in General Relativity is equivalent to a constant energy field with a very low energy density everywhere in the Universe. This means that the total amount of dark energy in the Universe has increased steadily since the Big Bang as the volume of the Universe has increased as it has expanded at the speed of light.

A cosmological constant or dark matter energy field, however, aren't the only possible explanations for what we observe. It could also be interpreted, for example, as a slight intrinsic curvature of space-time that is observable only at very long distance scales.

Recent observations, however, call into question whether observations are consistent with this energy field really being constant over time, or whether it actually changes is magnitude over time. Existing observations are only just barely good enough to distinguish between the different dark energy theories that have received serious consideration, and then, not to a high enough level of statistical significance to convince physicists and astronomers that one theory is definitely right and the other is definitely wrong. New telescopes, like the James Webb Space Telescope (JWST), will allow us to distinguish between the possibilities much more clearly in the years to come.

There are also credible questions about how accurate our estimate of the amount of dark energy in the Universe is that are directed at the methodology and error estimates in current efforts to determine this amount. If some important systemic error in the observations we are relying upon to quantify how much dark energy there is has been overlooked, this could result in big changes in its magnitude (e.g. the magnitude of dark energy effects might be half what they are estimated at by current methods).

Quite a few investigators also have tried to explain the phenomena attributed to dark energy and the phenomena attributed to cosmological inflation (e.g. the amount of homogeneity in the Universe at large scales) with a single theory.

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Klystron, vanhees71 and PeroK
Not that I don't understand your point, In fact, your words are very clear, My confusion is we know nothing about the dark matter more so dark energy yet we say Universe is almost 90% Dark Matter and energy.
To say "we know nothing about the dark matter more so dark energy" isn't quite correct. Based on observations of the cosmic microwave background radiation we have reason to believe that our universe is spatially flat. This fits perfectly to the analysis of supernovae data which together with CMB data show that the total energy density consists to ~70% of Dark Energy and to ~30% of matter including Dark Matter. That's in short what we call the Lambda-CDM model.

vanhees71
To say "we know nothing about the dark matter more so dark energy" isn't quite correct.
I think the point is that we don't have a good understanding of the properties and nature of what causes the phenomena attributed to dark matter and dark energy, in other words, their physical mechanisms. We can fit them to parameters in an abstract model, but that is about it.

Viable proposals for dark matter range from axion-like particle dark matter made of particles a thousand to millions of times less massive the neutrinos to primordial black hole candidates with masses on the order of asteroids. In some theories it is completely collisionless and doesn't interact at all with anything other than gravitationally, while in others dark matter interacts with other dark matter nearly as strongly as a short-range version of the electromagnetic force. In other theories, it even has slight non-gravitational interactions with ordinary matter (which are weaker than the weak force, but stronger than gravity). There are viable proposals to explain dark matter phenomena with gravitational modifications as well.

We know a lot about what dark matter is not. And, we have some rough outlines of how it behaves. But there is very little we can say definitively about what it is.

The situation is better with dark energy, and it isn't really fair to say that we know nothing about it, although the certainty of that knowledge shouldn't be overrated.

To a rough approximation, dark energy acts as a constant energy density per volume in the Universe in otherwise empty space, that grows as the size of the Universe grows.

This may not be exactly right. Indeed, there are lots of strong observational hints that it isn't exactly right, although none to the strict standards that astrophysicists would require to consider the plain vanilla cosmological constant in the GR equations hypothesis disproven. But, while there are a variety of explanations, mostly, they aren't terribly dissimilar to each other.

It is also worth noting that while the "70% of the mass-energy in the Universe is dark energy" is a pretty overwhelmingly way to think about it, that intuitively you would think would have a huge impact on daily life, dark energy effects are quite subtle. In terms of mass-energy per cubic meter it is estimated to be:

So, about the same mass-energy as thee and a half hydrogen atoms per cubic meter. This turns out to be negligible for calculations even at the scale of an entire galaxy, particularly because the effect cancel out at the scale since the distribution is the same in all directions.

By comparison, a cubic meter of water has a mass of 1000 kg and has about 3.33*1028 oxygen atoms and 6.66*1028 hydrogen atoms.

the effect cancel out at the scale since the distribution is the same in all directions
This is not quite correct. The main "effect" of dark energy is to make geodesics diverge, and that effect does not "cancel out" on any scale. It's just too tiny to matter in our models of the dynamics, since measurement errors are many orders of magnitude larger.

Klystron and vanhees71
This is not quite correct. The main "effect" of dark energy is to make geodesics diverge, and that effect does not "cancel out" on any scale. It's just too tiny to matter in our models of the dynamics, since measurement errors are many orders of magnitude larger.
Not wrong, but it is also the case that if dark energy were concentrated in big lumps that were widely separated, that it would have far greater local effects.

Zeal Faust
if dark energy were concentrated in big lumps that were widely separated
The usual name for something like this would not be "dark energy", it would be "exotic matter". It would indeed have much larger local effects than dark energy, which we don't see, hence we don't think exotic matter exists in any significant quantity.

vanhees71 and Jorrie