Dark Matter Real: Studies Confirm, Modifying Gravity Can't Work

[Janus]

Thanks for this. So...the DM/Gravity "force" is very weak leading to a highly likely BH "miss" at perigee and a conventional (if loose) "orbit" around the averaged gravitational galactic centers–all assuming DM can't be "compressed".

Whether DM enters a BH is thus a very low probability ie. inconsequential event. However, it seems of interest insofar as concerns DM & interaction with the BH Conversion Zone–BHCZ (Event Horizon–where baryons are converted into whatever is in BH). Is DM even "convertable"? I see other questions occupy much higher priority.

As for DM and DE: Are they like oil and water?

DM doesn't interact with the BH any differently than "normal" matter does in terms of gravity. The only difference is that "regular matter" can collide with other matter on its way in and and thus give up some of its kinetic energy and velocity which, in turn gives the BH gravity more time to act on it and deflect its path towards the BH. Barring any such interaction with other matter( and assuming no charge for the BH), a proton and a particle of DM starting on the same initial trajectory will follow the same path, either swinging around or crossing the event horizon, depending on their starting trajectory.

As far as DM crossing the event horizon is concerned, mass is mass, it doesn't matter if it is from baryonic matter, DM, or the mass equivalence of photons. Passing the event horizon itself doen't mean that anything is "converted" into anything else. The event horizon just marks a boundary which limits what information we can get from the other side.

 

[sector99]

DM doesn't interact with the BH any differently than "normal" matter does in terms of gravity. The only difference is that "regular matter" can collide with other matter on its way in and and thus give up some of its kinetic energy and velocity which, in turn gives the BH gravity more time to act on it and deflect its path towards the BH. Barring any such interaction with other matter( and assuming no charge for the BH), a proton and a particle of DM starting on the same initial trajectory will follow the same path, either swinging around or crossing the event horizon, depending on their starting trajectory.

As far as DM crossing the event horizon is concerned, mass is mass, it doesn't matter if it is from baryonic matter, DM, or the mass equivalence of photons. Passing the event horizon itself doen't mean that anything is "converted" into anything else. The event horizon just marks a boundary which limits what information we can get from the other side.

I'm having trouble with "mass is mass". DM teaches so much by being so "Dark". Not withstanding Chandra's x-ray images of hot gas presumed to be associated with DM.

 

[Orodruin]

Not withstanding Chandra's x-ray images of hot gas presumed to be associated with DM.

Are you referring to galaxy mergers such as the Bullet Cluster? The entire point there is that the hot gas is not associated with dark matter.

 

[sector99]

Are you referring to galaxy mergers such as the Bullet Cluster? The entire point there is that the hot gas is not associated with dark matter.

Here's the link:

https://www.nasa.gov/vision/universe/starsgalaxies/dark_matter_proven.html
Chandra's detector receives high energy photons, below claimed to be "purple" in hue. Photons aren't DM. Perhaps I've misinterpreted the text.

 

[Bandersnatch]

Chandra's detector receives high energy photons, below claimed to be "purple" in hue. Photons aren't DM. Perhaps I've misinterpreted the text.

The purple bits to the left and right show DM distribution - it's the lensing map mentioned in the description. The x-ray emission from gas are in red, in the centre.

 

[Orodruin]

Here's the link:

https://www.nasa.gov/vision/universe/starsgalaxies/dark_matter_proven.html
Chandra's detector receives high energy photons, below claimed to be "purple" in hue. Photons aren't DM. Perhaps I've misinterpreted the text.

View attachment 242080

If you had provided the link from the beginning, we would not have had to guess. Yes, that is the Bullet Cluster. No, the x-rays are not from dark matter. Rather, the entire point is that the mass distribution inferred from gravitational lensing (purple) does not match up with the x-ray emitting gas (pink).

 

[sector99]

"A purple haze shows DM flanking the Bullet Cluster".

The caption's intended impression remains that the "purple haze" is DM.

If you had provided the link from the beginning, we would not have had to guess. Yes, that is the Bullet Cluster. No, the x-rays are not from dark matter. Rather, the entire point is that the mass distribution inferred from gravitational lensing (purple) does not match up with the x-ray emitting gas (pink).

I provided the link on page 5 at the beginning.

Meanwhile, here is an excerpt clarifying the Clowe, et al finding:

The original link left the incorrect impression that DM was directly imaged as a flanking "purple haze"[1st link].

Clowe et al shows immediatly above that DM was subject to "weak lensing", while the hot red-orange baryonic "plasma distribution" was subject to strong lensing and spilt, to 8 sigma. The essentially spherical bluish "halo" is the (mostly unlensed) spherical DM. It's DM because it's mostly unlensed and didn't react as baryonic plasma.

Lesson: Don't accept an interpretation when you have the original.

 

[Orodruin]

The purple is not an x ray image. It is a mass distribution inferred from gravitational lensing. The x ray image is shown in pink.

The original link left the incorrect impression that DM was directly imaged as a flanking "purple haze"[1st link].

This is why you should not read popularised science for any purpose other than entertainment.

Clowe et al shows immediatly above that DM was subject to "weak lensing"

No. DM is the source of the weak lensing of the background galaxies. Not what is being subjected to gravitational lensing.

the hot red-orange baryonic "plasma distribution" was subject to strong lensing

No. The baryonic matter collided, giving rise to x ray emissions.

Lesson: Don't accept an interpretation when you have the original.

Also, don’t misrepresent the original.

 

[sector99]

Confusion stemmed from an official NASA release by a writer with the Chandra group who is much closer to the topic than I.

Your view is correct. I have discarded the popular DM misperception that DM can be seen and more carefully reread Clowe et al to fully agree with all points of your critique. As an admitted newbie I had no intention of misrepresenting anything.
***********
My initial DM query was simply to ask "What exactly are observers seeing when they detect DM"?

Do you have any problems with this answer?

What is seen with recent x-ray studies is an advancing, post collisional hot plasma bow shock wave consistent with being gravitationally dragged through an invisible, resistant, fluid-like substance whose presence is consistent with DM theory.

 

[Bandersnatch]

My initial DM query was simply to ask "What exactly are observers seeing when they detect DM"?

Do you have any problems with this answer?

What is seen with recent x-ray studies is an advancing, post collisional hot plasma bow shock wave consistent with being gravitationally dragged through an invisible, resistant, fluid-like substance whose presence is consistent with DM theory

No, this is still incorrect. The x-ray image of colliding plasma has nothing to do with being dragged through DM or any resistance it may pose, gravitational or otherwise. By itself, it does not show DM in any sense.
In the Bullet cluster one observes two clusters of galaxies, which as per the traditional theory of galaxy structure should have two components accounting for its mass - the stars, and the interstellar+intergalactic gas. The majority of the mass should be in the gas component.
The gas in either aggregation collides with the gas in the other (while the stars pass through), creating the x-ray image captured by Chandra's x-ray detectors, and showing the gas to have separated from the optically luminous stellar component.
This is expected regardless of any DM content and there's nothing in the paper about DM affecting the way the gas has separated.
The additional - and completely separate from x-ray detection - observation that allows DM to be inferred is the map of gravitational lensing, that shows where most of the mass in the colliding clusters is concentrated. In the no-DM paradigm, the lensing map should show the majority of the mass to be where the gas is.
Since the map shows the mass to be mostly where the stars are, not where the gas is, one can infer that there is some additional mass component, which is - like stars and unlike gas - collisionless, and - unlike stars or gas - invisible.
That's why the image credit in the bit cited in post #109 lists all those different observations. It's wrong to pick the x-ray one and say it shows DM. If anything, it's the lensing map that is the closest to 'showing' DM, since it tracks the gravitational signature.

 

[sector99]

No, this is still incorrect. The x-ray image of colliding plasma has nothing to do with being dragged through DM or any resistance it may pose, gravitational or otherwise. By itself, it does not show DM in any sense.
In the Bullet cluster one observes two clusters of galaxies, which as per the traditional theory of galaxy structure should have two components accounting for its mass - the stars, and the interstellar+intergalactic gas. The majority of the mass should be in the gas component.
The gas in either aggregation collides with the gas in the other (while the stars pass through), creating the x-ray image captured by Chandra's x-ray detectors, and showing the gas to have separated from the optically luminous stellar component.
This is expected regardless of any DM content and there's nothing in the paper about DM affecting the way the gas has separated.
The additional - and completely separate from x-ray detection - observation that allows DM to be inferred is the map of gravitational lensing, that shows where most of the mass in the colliding clusters is concentrated. In the no-DM paradigm, the lensing map should show the majority of the mass to be where the gas is.
Since the map shows the mass to be mostly where the stars are, not where the gas is, one can infer that there is some additional mass component, which is - like stars and unlike gas - collisionless, and - unlike stars or gas - invisible.
That's why the image credit in the bit cited in post #109 lists all those different observations. It's wrong to pick the x-ray one and say it shows DM. If anything, it's the lensing map that is the closest to 'showing' DM, since it tracks the gravitational signature.

Let me take another attempt to answer my 1st query: "What exactly are observers seeing when they detect DM"?

(1) Galactic baryons showing anomalously high relative speed infers DM. (Normal stellar velocities were recently asserted to reveal an absence of DM)
(2) Detectable gas displaced from galactic stellar components infers DM. (This is the 2nd sentence and 1st assertion in the Clowe, et al paper) The definitial problem is widely varying detectable gas morphologies.

If it's possible to make this answer simpler I'll be happy to keep listening.

 

[mfb]

(Normal stellar velocities were recently asserted to reveal an absence of DM)

In two galaxies. Meanwhile they show the presence of dark matter in thousands of others and on many different scales. It will be interesting to figure out how these two galaxies formed with nearly no matter.
(3) gravitational lensing shows that there is additional matter
(4) the cosmic microwave background shows that there is additional matter

 

[sector99]

Thank you for this.

(assembled & quoted from above)
Let me take another attempt to answer my 1st query: "What exactly are observers seeing when they detect DM"?

(1) Galactic baryons showing anomalously high relative speed infers DM. (Normal stellar velocities were recently asserted to reveal an absence of DM)
(2) Detectable gas displaced from galactic stellar components infers DM. (This is the 2nd sentence and 1st assertion in the Clowe, et al paper) The definitial problem is widely varying detectable gas morphologies.
(3) gravitational lensing shows that there is additional matter
(4) the cosmic microwave background shows that there is additional matter

Provisional (5*)
Galaxy spiral kinematics. Asked another way: Could spirals have formed without DM?

 

[sector99]

Off Topic FYI:

 

[Janus]

Provisional (5*)
Galaxy spiral kinematics. Asked another way: Could spirals have formed without DM?

The galactic spirals are not due to there being significantly more material in the arms than between them, they are regions where there are more hot younger stars. They are basically caused by density waves traveling through the galaxy. This increases star formation. This includes massive bright stars with short lifetimes. This makes the spiral arms look brighter. But these bright stars burn out quickly, and so by the time the wave passes, only small dim stars are left and these are what make the large population of stars in the "gaps". There is still a lot of material in there, it's just dimmer. So spiral arms aren't structures of which stars are permanent residents of, but rather, stars move in and out of the spiral arms.

 

[sector99]

Thanks. This raises a query about DM density waves. I'm guessing that since DM somehow can't be "compressed" there can't be any DM density waves thus the spiral appearance (which doesn't arise from kinematic rotation-including especially the barred spirals) is due solely to baryonic matter?

 

[mfb]

Dark matter can be compressed easily. What would stop it? It is collisionless (at least to a good approximation).

 

[Ken G]

Dark matter would be hard to compress externally by some kind of wall, but there aren't walls like that in any astrophysical context (other than perhaps magnetic fields, but those aren't active in, say, spiral density waves either). Collisionless does not mean low pressure, so can have high pressure and exhibit the same resistance to compression as any high pressure gas. Indeed, spiral density waves don't separate the stars from the gas, even though the former is collisionless and the latter is collisional. So something a little different than a gravitationally induced density wave must going on in the Bullet Cluster.

Gas pressure is the momentum flux density of the particles, so only requires velocities that are high and isotropic. In short, the ideal gas law. It's true that collisions are useful for maintaining the isotropic velocities (that's really all collisions do in regard to gas pressure), but there must be some other way that dark matter does it because dark matter is normally assumed to be at a temperature. I think it must be its history of obeying the cosmological principle which must ultimately be the source of the isotropy of its velocities. Honestly I am not sure why dark matter is always assumed to be thermalized, and even often treated as isothermal. Clearly the dark matter particles interacting in the Bullet Cluster has velocities that "remember" which cluster they came from, so are not thermalized, yet the Milky Way dark matter is generally assumed to be thermalized. It must have to do with the history.

The confusion about pressure and compressibility in collisionless gases probably traces to the fact that introductory sources often confuse pressure with forces on boundaries next to the gas, which requires collisions with the boundary (though still not between the particles themselves). But the force from/on a wall is actually nothing more than the action/reaction involved in the mundane "normal force" seen in so many other contexts, so is not pressure any more than standing on a bathroom scale is gravity, but it is a good way to measure pressure of gas that isn't dark matter. I don't know why so many sources like to think of gas pressure as a force on/from a wall, as it is more usefully and more flexibly thought of as a force on the gas itself, stemming from the gas itself, stemming from the momentum flux density within the gas. And even though the fluid approximation is made much easier to assume by collisions, it is used for dark matter also. But yes, the Bullet Cluster dark matter is more easily understood not in a fluid picture!

 

[sector99]

DM not being "compressed" or, collected by galactic center gravitation was what I meant to say.

On the characteristic of being collisionless–does this feature reduce exotic bosonic/hadronic matter as a DM candidate?

 

[jocarren]

I have a methodological objection about DM. Such objection doesn't mean that i reject the DM hypothesis nor all of the observations that support it, but to express concern about the following:

1.- The difficulty for detection.
2.- Reluctancy to postulate new physics.

Since the existence of DM is inferred from it's gravitational effect on galactic motion and there is no other interaction (internal nor external) it is pretty much defined by this two aspects.

This raises the question: how can DM be observed if not by it's defining qualities?

Up until this point, most of the observation efforts are focused on mapping it's gravitational interaction (mainly through lensing). The remaining effort is focused on finding correlations between expected behavior of particles both hypothetical and within the standard model, with no success as far as I know.

Problem is, as difficult as proving it's existence is, maybe it's even more difficult to disprove it. Since the only thing that can be observed is the very thing that defines the subject of observation: Gravity and non interaction.

The way i see it, the unfalsifiability issue is not inherent to the claim that something is causing the phenomena observed, but emerges from an excessively broad definition of DM. Efforts should be made not only to observe it, but to narrow it's definition so the verification can be methodologically simpler.

About the reluctancy towards new physics, i don't mean to say that GR is wrong and the subsequent cosmological models are too, everything seems to point out that they are correct (and i personally subscribe to that notion), but this sort of discrepancy between the observed phenomena and the proven theory claims for a paradigm shift or at the very least (which i believe is the case here) a fresh interpretation of mainstream physics.

But, as i said before, this is not about disproving DM, it's about the concern that a methodological issue can undermine it's verification.

 

[Orodruin]

I have a methodological objection about DM. Such objection doesn't mean that i reject the DM hypothesis nor all of the observations that support it, but to express concern about the following:

1.- The difficulty for detection.
2.- Reluctancy to postulate new physics.

Since the existence of DM is inferred from it's gravitational effect on galactic motion and there is no other interaction (internal nor external) it is pretty much defined by this two aspects.

I believe you are, to a large degree, chasing after a red herring. If it was not difficult to detect dark matter, it would have been observed a long time ago and so it is rather a sign of the times we live in.

When it comes to reluctancy to postulate new physics in the dark matter community, it is simply untrue. If you had spent any significant amount of time reading research articles about dark matter and dark matter models, you would be aware of how eager dark matter theorists are to dream up new ways of postulating physics beyond the standard model that could explain the dark matter observations.

This raises the question: how can DM be observed if not by it's defining qualities?

This depends on what dark matter actually is, which we have a large number of theories about. For most of those theories, there are accompanying ways of looking for dark matter in other ways. Some are being actively pursued by the experimental community.

Up until this point, most of the observation efforts are focused on mapping it's gravitational interaction (mainly through lensing). The remaining effort is focused on finding correlations between expected behavior of particles both hypothetical and within the standard model, with no success as far as I know.

Problem is, as difficult as proving it's existence is, maybe it's even more difficult to disprove it. Since the only thing that can be observed is the very thing that defines the subject of observation: Gravity and non interaction.

There is absolutely no question about the fact that something is going on with respect to gravitational phenomena. This something is generally well described by cold dark matter. If it looks like a duck and quacks like a duck ... You might as well call it a duck.

The way i see it, the unfalsifiability issue is not inherent to the claim that something is causing the phenomena observed, but emerges from an excessively broad definition of DM. Efforts should be made not only to observe it, but to narrow it's definition so the verification can be methodologically simpler.

Again, this just shows that you are not familiar with how front-line research is being done within the dark matter community. Dark matter is not a single thing, it is rather a phenomenon that can possibly be explained by a particle with some particular properties. The point is not narrowing the definition of this phenomenon, it is to investigate models where the phenomenon arises and consider what other effects would arise from those models. This is precisely what is being done.

About the reluctancy towards new physics, i don't mean to say that GR is wrong and the subsequent cosmological models are too, everything seems to point out that they are correct (and i personally subscribe to that notion), but this sort of discrepancy between the observed phenomena and the proven theory claims for a paradigm shift or at the very least (which i believe is the case here) a fresh interpretation of mainstream physics.

Again, saying that the dark matter community is reluctant towards new physics could not be further from the truth.

 

[Ken G]

We should probably accept it as natural when a new theory exhibits growing pains. We were spoiled with general relativity, which went from formulative to well-verified in just a few decades. Dark matter has already taken much longer than that, and will likely exhibit further growing pains going forward-- it's not a cause for any panic. It hasn't even generated a Nobel prize yet, because the progress has been slow and rather incremental. Dark energy did generate a Nobel prize, but clearly has more growing pains to go through as well. We simply need to press forward on efforts to hone and test these theories, keeping open all alternatives but also investing in making gains in whatever area seems to be the most productive. At present, that's WIMP type theories.

 

[jocarren]

I believe you are, to a large degree, chasing after a red herring. If it was not difficult to detect dark matter, it would have been observed a long time ago and so it is rather a sign of the times we live in.

When it comes to reluctancy to postulate new physics in the dark matter community, it is simply untrue. If you had spent any significant amount of time reading research articles about dark matter and dark matter models, you would be aware of how eager dark matter theorists are to dream up new ways of postulating physics beyond the standard model that could explain the dark matter observations.

That is tranquilizing.

This depends on what dark matter actually is, which we have a large number of theories about. For most of those theories, there are accompanying ways of looking for dark matter in other ways. Some are being actively pursued by the experimental community.

There is absolutely no question about the fact that something is going on with respect to gravitational phenomena. This something is generally well described by cold dark matter. If it looks like a duck and quacks like a duck ... You might as well call it a duck.

It could be a goose (i think DM it's a lousy name)

Again, this just shows that you are not familiar with how front-line research is being done within the dark matter community. Dark matter is not a single thing, it is rather a phenomenon that can possibly be explained by a particle with some particular properties. The point is not narrowing the definition of this phenomenon, it is to investigate models where the phenomenon arises and consider what other effects would arise from those models. This is precisely what is being done.

Narrowing down the definition it's important in the "non-interaction" part, if the definition of something is "you can't observe it", experimental work becomes increasingly hard.

Again, saying that the dark matter community is reluctant towards new physics could not be further from the truth.

Again, good to know.

 

[Ken G]

The "W" in WIMP is "weakly", not "non." Most people regard neutrinos as dark matter, for example, and it was difficult, but not impossible, to verify them.

 

[sector99]

["Again, saying that the dark matter community is reluctant towards new physics could not be further from the truth."]

Well said. The effort may well trend towards a hybrid solution–given the extreme complexity of SDSS large-scale mapping. Starting with the question: Do DM constituents have to be matter?

As a total newbie, I stumbled upon Fahr/Heyl and their recent work involving abundant invisible possible candidates including photons plus vacuum energy & mass density under expansion scenarios:

The idea that CMB photons retain their 3000K creation energy until detection caught me by surprise–as did the theoretical standing wave mode involved in redshift (5,7 & 8 above).

 

[PeterDonis]

Reluctancy to postulate new physics.

I don't understand this objection: postulating dark matter is postulating new physics--it is postulating the existence of particles (more precisely, quantum fields) other than those that appear in the Standard Model of particle physics.

It is true that postulating dark matter is not postulating new gravitational physics--the dark matter hypothesis assumes that our best current theory of gravity, General Relativity, is correct, by contrast with MOND and similar hypotheses, which do not. But this is not a matter of postulating new physics vs. not postulating new physics; it's only a matter of where one prefers to put the new physics--new particles/fields vs. a new theory of gravity.

 

[Orodruin]

i think DM it's a lousy name

That's your prerogative. It does not change anything of substance whether you like the name or not.

Narrowing down the definition it's important in the "non-interaction" part, if the definition of something is "you can't observe it", experimental work becomes increasingly hard.

It is not part of the definition that it does not interact. You have simply somehow gotten the wrong ideas into your mind.

 

[jocarren]

I apologize, i don't mean to cuestion the work of physicist on the subject, just to place my concerns (wich can be failures on my behalf).
What I mean by "a lousy name" is that is misleading: about "dark", it's low (or no) interaction can be better described as "translucid", and "matter" (wich is the good part of the name, in my opinion) is not completely accurate because there is no certainty that the effect observed is indeed matter (although is the most plausible explanation).

 

[Ken G]

I tell students that the term "dark" means like the "dark ages," more along the lines that we are "in the dark" about it-- just like "dark energy." But the term still causes confusion, and there are always students who take the sucker answer on a multiple choice test that dark matter is what you see when you are actually looking at dust lanes in a galaxy. So yes, a better name would have been possible, but what else is new in astronomy? (Planetary nebulae, white dwarfs vs. main-sequence dwarfs, Big Bang, stars look brighter as their "magnitude" goes down, OBAFGKM, the list goes on and on.) A bad name can be used as a teaching moment, in the process of understanding its flaws and the history of why it came to be called that.

 

[phinds]

As for DM and DE: Are they like oil and water?

No. Oil and water are both liquids. DM and DE are more like fish and bicycles. They have absolutely nothing to do with each other.

 

[ohwilleke]

No. Oil and water are both liquids. DM and DE are more like fish and bicycles. They have absolutely nothing to do with each other.

Maybe, maybe not. There are many extant theories that attempt to explain both in an integrated fashion.

For example, Chaplygin gas models such as those discussed at https://arxiv.org/abs/1904.07510, or "Dark energy and dark matter unification from dynamical space time: observational constraints and cosmological implications" https://arxiv.org/abs/1904.05762, to name a couple of papers just in the last month.

There have also been modified gravity proposals to integrate dark matter and dark energy. Again, looking just at examples of papers released this month, these include f(T) gravity https://arxiv.org/abs/1904.09897 and (less recently) conformal gravity https://arxiv.org/abs/1208.4972.

One motivation for a unified explanation is the "cosmic coincidence problem" (i.e. that the total amount of dark matter and the total amount of dark energy in the universe are of the same order of magnitude). See, discussing this "problem" (scare quotes in the original) https://arxiv.org/abs/1410.2509.

 

[ohwilleke]

It is not part of the definition that it does not interact. You have simply somehow gotten the wrong ideas into your mind.

The lambdaCDM definition includes "almost collisionless" which isn't inconsistent with not interacting at all non-gravitationally, but also doesn't require that it not interact non-gravitationally. (Obviously, it must interact gravitationally since that is how we observe it.) Generally "almost collisionless" is defined operationally as not significantly more strongly interacting with ordinary matter than neutrinos.

 

[ohwilleke]

I don't understand this objection: postulating dark matter is postulating new physics--it is postulating the existence of particles (more precisely, quantum fields) other than those that appear in the Standard Model of particle physics.

It is true that postulating dark matter is not postulating new gravitational physics--the dark matter hypothesis assumes that our best current theory of gravity, General Relativity, is correct, by contrast with MOND and similar hypotheses, which do not. But this is not a matter of postulating new physics vs. not postulating new physics; it's only a matter of where one prefers to put the new physics--new particles/fields vs. a new theory of gravity.

Just to complicate matters further, many of the leading dark matter particle theories (e.g. self-interaction dark matter or SIDM) theories, introduce not only "new physics" particle/quantum field, but also introduce a "new physics" fifth force with its own (usually a Yukawa force with a light but massive carrier boson).

 

[ohwilleke]

Most people regard neutrinos as dark matter

I'm not convinced that this is accurate.

 

[ohwilleke]

At present, that's WIMP type theories.

WIMP theories are among the least viable of dark matter particle theories outstanding and have been ruled out over very wide areas of parameter space. The originally most popular WIMP theories, those in which the lightest supersymmetric particle serves as the dark matter candidate, are virtually entirely ruled out.

Some of the most promising dark matter particle theories these days are those with axion-like dark matter particles, and those with self-interacting dark matter particles with this fifth dark matter force mediated by a massive carrier boson.