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I'm a complete layperson. As I understand, dark matter theoretically only interacts with the gravitational force, and doesn't interact with the other three fundamental forces: weak nuclear force, strong nuclear force, and electromagnetism.

Those are my understandings going in. If I'm wrong, please correct me. I've done some googling, and I haven't found anything confirming or denying that dark matter is affected by either of the fundamental nuclear forces.

So since dark matter only interacts with gravity, what causes any dark matter particle to be repelled from another? If they can pass freely through each other, and they are gravitationaly attracted to each other, why don't such particles clump together in a single 'point' in space?

It seems to me that particles occupying a single 'space' are philosophically not distinct particles, but I don't know how actual physics would play into this.

Edit This article, author's credentials unknown, but implicitly claims to be a physicist or astronomer, says "...[P]hysicists generally take all dark matter to be composed of a single type of particle that essentially interacts only through gravity."

Edit 2 The author is this Lisa Randall, "Professor of Science on the physics faculty of Harvard University."

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    $\begingroup$ Think about what it means when you say "clump"... $\endgroup$
    – dmckee --- ex-moderator kitten
    Oct 27, 2015 at 18:41
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    $\begingroup$ @dmckee I'm not well-versed in physics, but I suppose your asking me if I'm thinking of atoms and molecules when I say 'clump', which I understand is caused by strong nuclear force. But, by 'clump', I mean in tight gravitational orbit of each other. And by logical extension, if they pass through each other, can their orbits be so tight that they occupy the same space, like a singularity? If that's not what you're asking me, I could use some exposition :) $\endgroup$
    – user151841
    Oct 27, 2015 at 18:46
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    $\begingroup$ You could ask the same question about 'normal' matter: why doesn't it all 'clump' together? Also, you state DM doesn't interact with the three other fundamental forces but do we really know that, considering how little we know about DM? $\endgroup$
    – Gert
    Oct 27, 2015 at 18:51
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    $\begingroup$ The homogeneity of the early universe. As far as we can tell based on measurements of the cosmic microwave background all "stuff", including dark matter, was pretty evenly distributed when the universe "began". Since then the universe is coming to thermal equilibrium, which for gravitating objects involves agglomeration. On the other hand dark matter is probably not stable, so it will decay as it clumps and this will, once again, homogenize the universe, but this time into a very cold, evenly distributed state of photons. $\endgroup$
    – CuriousOne
    Oct 27, 2015 at 19:07
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    $\begingroup$ Related/duplicate: physics.stackexchange.com/q/174977 $\endgroup$
    – Kyle Oman
    Oct 28, 2015 at 16:59

7 Answers 7

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Great question. Observations show that Dark Matter (DM) only noticeably interacts gravitationally, although it's possible that it may interact in other ways "weakly" (e.g. in the 'WIMP' model --- linked). Everything following has no dependence on whether DM interacts purely/only gravitationally, or just predominantly gravitationally --- so I'll treat it as the former case for convenience.

Observable matter in the universe 'clumps' together tremendously: in gas clouds, stars, planets, disks, galaxies, etc. It does this because of electromagnetic (EM) interactions which are able to dissipate energy. If you roll a ball along a flat surface it will slow down and eventually stop (effectively 'clumping' to the ground), because dissipative forces (friction) are able to transfer its kinetic energy away.

On the other hand, imagine you drill a perfect hole, straight through the center of the Earth, and you drop a ball down it. (Assuming the hole and the earth are perfectly symmetrical...) the ball will just continually oscillate back and forth from each side of the earth to the other --- because of conservation of energy. Just like a frictionless pendulum (no rubbing, no air resistance). This is how dark matter interacts, purely gravitationally. Even if there was no hole through the center of the earth, the DM will just pass straight through and continue to oscillate back and forth, always reaching the same initial height. To zeroth order, dark matter can only 'clump' as much as its initial energy (obtained soon after the big-bang) allows. One example of such a 'clump' is a 'Dark Matter Halo' in which galaxies are embedded. DM Halos are (effectively) always larger than the normal (baryonic) matter inside them --- because the normal matter is able to dissipate energy and collapse farther.

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    $\begingroup$ "Assuming the hole and the earth are perfectly symmetrical..." - also assuming the Earth does not rotate (or the hole was drilled perfectly in line with its rotation axis). $\endgroup$
    – John Dvorak
    Oct 27, 2015 at 19:52
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    $\begingroup$ @JanDvorak and that the moon and sun don't exist, and .. and :) $\endgroup$
    – Steve
    Oct 27, 2015 at 20:57
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    $\begingroup$ @JanDvorak ... and we can assume the ball actually behaves as a proper spherical cow would, for purposes of analyses. That way the experiment will be reproducible in undergrad physics classes later. Most undergrad physics courses have limited access to balls but access to a nearly limitless amount of spherical cows. $\endgroup$
    – Cort Ammon
    Oct 27, 2015 at 21:15
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    $\begingroup$ So dark matter doesn't interact hence it cannot clump into itself? What about a black hole? If dark matter enters the event horizon, can it pass through? What type of mass is added to the black hole? $\endgroup$
    – Paul
    Oct 27, 2015 at 22:06
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    $\begingroup$ @Paul, DM does interact gravitationally --- so it should interact with black holes just like any other stuff. $\endgroup$
    – DilithiumMatrix
    Oct 27, 2015 at 23:42
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Because the dark matter does not interact a lot, there is no mechanism that would slow it down quickly. When a dark matter particle is falling towards some gravitational center, it is speeding up, then it flies through the periapsis and continues away into the distance. Normal matter clumps into planets, because it is slowed down by interactions / collisions. Dark matter does not collide and cannot deposit energy. It stays on elliptical orbits with very large axes and there is no way how to shrink the ellipse. Normal matter can shrink its orbital path by collisions, but not dark matter.

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    $\begingroup$ How is our galaxy held together then? Doesn't this require clumping of dark matter towards the center of the galaxy? $\endgroup$
    – Peter Mortensen
    Oct 28, 2015 at 23:59
  • $\begingroup$ @PeterMortensen Quite the contrary - it really can't clump much. Imagine a dark matter particle entering our galaxy at random - what happens? Pretty much exactly the same thing that happens with any rogue star - depending on luck, it might shoot out even faster than it came in, it might find a nice elliptical orbit in the galaxy... To make it clump in the center, you would need a series of pretty specific interactions with surrounding matter (dark or not) - and in each of those, one body is accelerated, while the other is slowed down; on average, momentum and energy is conserved. $\endgroup$
    – Luaan
    Oct 29, 2015 at 8:30
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    $\begingroup$ @PeterMortensen On the other hand, with EM interactions, there's a prevalence of losing energy to the universe - a lot of EM interactions involve shooting photons out of the galaxy, taking energy with them. For example, star systems form from gas clouds this way - the EM collisions make their particles lose energy over time as light. In the galaxy at large, gravity seems to dominate almost absolutely - which is why normal matter behaves pretty much the same as dark matter on galactic scales. But collisions, supernovae, solar wind... all those are almost exclusively EM interactions. $\endgroup$
    – Luaan
    Oct 29, 2015 at 8:34
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    $\begingroup$ @PeterMortensen Of course, there's a tiny amount of energy radiated away in gravitational interactions as well - but again, since gravity is so incredibly weak, this is only really noticeable in situations like two neutron stars orbiting each other very closely. It should lead to some clumping - but I can't really put a number on that. It might very well be enough for the falloff we expect theoretically (a simple r squared dependence). But we don't really know much - if there's a galactic halo of DM, for example, it might account for some of the "missing" energy. Observing DM is hard :D $\endgroup$
    – Luaan
    Oct 29, 2015 at 8:40
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At this point we know a lot more about what dark matter is not, than what it is. It does not interact via the electromagnetic force, and interaction via the strong force is also unlikely. Interaction via the weak force is still an active area of research (See here).

To understand why dark matter does not form clumps, imagine two particles of dust whizzing through space at high speed toward each other. They get close together but just narrowly avoid a head-on collision before going off in different directions. For a moment, when they were very close together, the pull of gravity between the two objects was at its strongest, but the particles were travelling too fast for the small gravitational pull to hold them together.

Now imagine a different scenario where the two dust particles collide head-on (which happens via the electromagnetic force). Now that the two particles have lost energy through heat, the gravitational pull between the particles can keep them held together in a clump. Soon, a third dust particle comes along and collides into this clump of dust, loses its kinetic energy, and becomes bound to the clump as well. As the clump of dust grows, more particles collide with it and it continues to grow larger and larger, eventually into a planet or star.

Dark matter rarely bumps into itself (or other matter), so it is almost always like the first case, rather than the second. Millions of dark matter particles are passing through you right now without hitting anything. Since it is so hard for them to get rid of their kinetic energy, they tend to not get bound up into clumps.

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  • $\begingroup$ When you say "[d]ark matter rarely bumps into itself (or other matter)", you mean it rarely passes through another particle? $\endgroup$
    – user151841
    Oct 28, 2015 at 16:23
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    $\begingroup$ Would you care to cite a source to "millions of dark mater particles ARE passing through you..." - just curious. $\endgroup$
    – Mindwin Remember Monica
    Oct 28, 2015 at 16:45
  • $\begingroup$ @user151841 Dark matter is constantly passing through regular matter, but rarely does it ever actually collide. $\endgroup$
    – Chris
    Oct 28, 2015 at 18:21
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    $\begingroup$ @Mindwin See paragraph 2 under heading 1: cosmology.berkeley.edu/preprints/cdms/9809009.pdf $\endgroup$
    – Chris
    Oct 28, 2015 at 18:22
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    $\begingroup$ @user151841 Exchange momentum with another particle via the weak nuclear force $\endgroup$
    – Chris
    Oct 28, 2015 at 18:36
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If you consider dark matter to be in the form of massive particles that have kinetic energy but only interact gravitationally, then there is a simple way to look at this.

If the particles start off in a configuration where their total energy (the sum of positive kinetic energy and negative gravitational potential energy) is zero; then they are on the cusp between being gravitationally bound or unbound.

To make things "clump" you need to make their total energy negative. The only way you can do this is to remove kinetic energy from the system.

With normal matter this is done through electromagnetic interactions, which turn the kinetic energy of normal matter (protons, electrons etc.) into photons, which then escape from the system. Since these kinds of interactions do not occur for dark matter (by definition), then there is no way to get rid of kinetic energy and so the dark matter remains as a large "halo" around gravitationally clumping ordinary matter.

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Dark matter might be matter which has no protons or neutrons, more like pure energy than the kind of matter which is familiar to us ... kind of like the "GEONs" which John Archibald Wheeler proposed, speculatively, many years ago (see his book Geons, Black Holes & Quantum Foam for details).

Because it contains energy, and because energy is gravitationally equivalent to mass, it has gravitational-interactions with ordinary matter, so it provides the extra mass necessary to prevent a galaxy from flying apart as it rotates.

But, because it contains no protons or neutrons, it can't collapse down and form stars, nor can it interact with photons, for the same reason; so it might be that the only way we can detect it is through its gravitational interactions.

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    $\begingroup$ This seems to be more of a speculative answer than anything else; there has been some research done on dark matter, and that is what the question is asking about. While this is an interesting answer, please try to stick to known facts, not personal theories. $\endgroup$
    – auden
    Jun 26, 2016 at 18:40
  • $\begingroup$ HI: I'm new here, and not sure if this is the right place for this kind of question ... I would like to message heather, privately, and don't know how to do this ... can someone explain how ? $\endgroup$
    – PERFESSER CREEK-WATER
    Jun 27, 2016 at 17:20
  • $\begingroup$ There is no private message system. You can @-tag people who have interacted with posts (commented or edited) in the comments to the posts they have interacted with posts, but it happens in public. $\endgroup$
    – dmckee --- ex-moderator kitten
    Jul 14, 2016 at 21:57
  • $\begingroup$ @heather Hi Heather: my answer is not a personal theory, but based on the work of a gentleman who studied at Cornell under Bethe + Morrison + Feynman, and received his PhD from there in 1953 ... However, knowing that his explanation of dark-matter is "non-standard", I'm afraid that if I post any more details, it will result in other site-users "dinging" my reputation ... is there any way to discuss a non-standard theory or model or idea without getting "dinged" ?? $\endgroup$
    – PERFESSER CREEK-WATER
    Jul 16, 2016 at 18:49
  • $\begingroup$ @PERFESSERCREEK-WATER, I think if you say what you just told me, no one will "ding" your reputation....however, I think there was a specific answer the OP wanted and you didn't really give it as this seems to be a theory that is very unknown (according to your comment) and is not the mainstream explanation on dark matter. $\endgroup$
    – auden
    Jul 16, 2016 at 20:04
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If dark matter is a spin-1/2 fermion with a two fold Wigner degeneracy, then Pauli exclusion principle would prevent a gravitational collapse. Details depend on mass of the particle. Ref, Nuclear Physics B 987 (2023) 116092.

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Because on their way to that hypothetical single point, they would pass a whole load of normal matter that would disturb their journey. You can more or less take non-gravitational forces out of the equation entirely and ask the same question of normal matter. That being said, the theory goes that (ultimately) everything will end up coalescing, and dark matter would be a participant in that. It just takes a while.

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