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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."

Qmechanic
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user151841
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    Think about what it means when you say "clump"... – dmckee --- ex-moderator kitten Oct 27 '15 at 18:41
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    @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 :) – user151841 Oct 27 '15 at 18:46
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    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? – Gert Oct 27 '15 at 18:51
  • @Gert I don't understand it fully, but I believe (subject to correction of course) that it clumps together the way it does, into atoms and molecules, because of strong nuclear force and electromagnetism, respectively. As for how little we know, I don't know why it isn't explicitly stated that "we don't know of dark matter interacts with the fundamental nuclear forces" rather than "only interacts with gravity". That's sort of what my question is. – user151841 Oct 27 '15 at 18:56
  • On very large scales dark matter does "clump", with the size of these lumps being given by the temperature of the dark matter. Eventually, when the universe is extremely cold, these lumps will be much smaller than they are now, but that's going to take a very long time. – CuriousOne Oct 27 '15 at 18:56
  • @CuriousOne So what cause[d/s] dark matter to separate in the first place? Are the particles separate from being flung by the Big Bang, or does the pull of baryonic matter keep them from forming larger clumps? – user151841 Oct 27 '15 at 19:01
  • Possible duplicate: http://physics.stackexchange.com/q/46634/2451 and links therein. – Qmechanic Oct 27 '15 at 19:01
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    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. – CuriousOne Oct 27 '15 at 19:07
  • When I asked about the word "clump" I was hope you would envision two small objects flying towards each other and colliding. Then you have to ask, do the fly apart again or do they stick? And to answer that you have to ask about what forces are responsible for their sticking. But if DM doesn't interact then there are no force to cause sticking. Mind you, the favorite experimental candidate is WIMP-like ad so does participate in the weak interaction, but that's a preference not a fact. – dmckee --- ex-moderator kitten Oct 27 '15 at 19:26
  • @dmckee As I tried to indicate earlier, I am a complete noob with physics, and I really don't know why two small objects stick together. I had thought what kept them from collapsing into a singularity was strong and weak nuclear force. But as far as dark matter, I was imagining solar systems, galaxies, stars, and black holes, which I understand 'clump' together because of gravity (particles in orbit of each other rather than smacking into each other) – user151841 Oct 27 '15 at 19:52
  • @dmckee so when gravity is strong enough to overwhelm the other forces that affect baryonic matter, the matter collapses into a black hole. Since (in some theories) DM is only affected by gravity, I didn't understand what would keep it from collapsing into a black hole also. – user151841 Oct 27 '15 at 20:00
  • "so when gravity is strong enough to overwhelm the other forces that affect baryonic matter, the matter collapses into a black hole." You are trying to move forward too far and too fast. First figure out why ordinary matter goes from clouds of gas and dust into perfectly ordinary objects like stars and planets. Don't imagine that black holes have special properties or require special processes because they started out as quite ordinary accumulations of matter. Then you can ask about what make dark matter behave differently than the stuff you're used to. – dmckee --- ex-moderator kitten Oct 27 '15 at 21:28
  • @dmckee I had always thought it was gravity, but apparently I have more to learn : ) – user151841 Oct 28 '15 at 02:07
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    Related/duplicate: http://physics.stackexchange.com/q/174977/ – Kyle Oman Oct 28 '15 at 16:59
  • I don't know if this adds anything but https://en.wikipedia.org/wiki/Dark_matter_halo suggests that it DOES clump. Only it "influences the universe's large-scale structure" (from: https://en.wikipedia.org/wiki/Dark_matter). So the great distances over which it is clumped make it seem like its not. – Len Feb 07 '18 at 00:06

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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.

DilithiumMatrix
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    "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). – John Dvorak Oct 27 '15 at 19:52
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    @JanDvorak and that the moon and sun don't exist, and .. and :) – Steve Oct 27 '15 at 20:57
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    @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. – Cort Ammon Oct 27 '15 at 21:15
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    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? – Paul Oct 27 '15 at 22:06
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    @Paul, DM does interact gravitationally --- so it should interact with black holes just like any other stuff. – DilithiumMatrix Oct 27 '15 at 23:42
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    So because no friction (or other interaction) Dark Matter particles describe an ideal N-body system with point masses. – ratchet freak Oct 28 '15 at 00:08
  • I don't that this answer is right. Assuming that dark matter is a particle it interacts with our observable universe only through gravity. However the dark matter particles themselves interact via some weak undefined interaction. – MaxW Oct 28 '15 at 01:38
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    @ratchetfreak exactly! That's one of the reasons why dark matter only simulations like 'Millennium' can have such large volumes and high resolutions --- N-Body simulations are far simpler and easier than including gas (etc). – DilithiumMatrix Oct 28 '15 at 03:13
  • A process called "dynamical friction" can however apply to dark matter. This can e.g. drag subhaloes to the centre of main haloes. – xioxox Oct 28 '15 at 15:48
  • @MaxW The weak force is called "weak" for a reason -- it's much weaker than the EM force and operates over a shorter range. This means that it's much, much, MUCH harder for WIMPs than for normal matter to effectively collide with one another and shed kinetic energy. Not that it doesn't happen at all, but it doesn't happen enough. – David Moles Oct 28 '15 at 22:38
  • +1 BTW for the note at the end about the relative sizes of DM halos and their baryonic matter counterparts -- I'd heard this explanation before but not that logical extension of it. – David Moles Oct 28 '15 at 22:39
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    @CortAmmon When you're done with your spherical cow, I'll cook myself up some... ground round. bows – corsiKa Oct 29 '15 at 18:08
  • The gravitational interaction with ordinary matter wouldn't make dark matter, very slowly, to loose energy and in the end collapse? The tidal locking of two objects is an example of this but with ordinary matter. Something similar could occur with dark matter while it interacts with ordinary matter. – user171780 Jul 31 '18 at 01:42
  • @user171780 tidal locking only happens because of EM interactions within each body. E.g. the tides, etc. point masses that only interact gravitationally can't lose energy that way. What would that even mean? How would it happen? – Vectorjohn Aug 10 '21 at 16:53
  • Is it possible that gravitational waves are providing the dissipation needed for dark matter collapse instead of EM forces? – FrogOfJuly Jul 08 '23 at 13:04
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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.

mpv
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    How is our galaxy held together then? Doesn't this require clumping of dark matter towards the center of the galaxy? – Peter Mortensen Oct 28 '15 at 23:59
  • @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. – Luaan Oct 29 '15 at 08:30
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    @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. – Luaan Oct 29 '15 at 08:34
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    @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 – Luaan Oct 29 '15 at 08:40
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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.

Chris
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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.

ProfRob
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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.

auden
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PERFESSER CREEK-WATER
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    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. – auden Jun 26 '16 at 18:40
  • 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 ? – PERFESSER CREEK-WATER Jun 27 '16 at 17:20
  • 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. – dmckee --- ex-moderator kitten Jul 14 '16 at 21:57
  • @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" ?? – PERFESSER CREEK-WATER Jul 16 '16 at 18:49
  • @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. – auden Jul 16 '16 at 20:04
  • @heather you said: "this seems to be a theory that is very unknown (according to your comment) and is not the mainstream explanation on dark matter. – heather" True: I started a chat-room to talk about it, and am going there now to see if anybody left any messages there !! – PERFESSER CREEK-WATER Jul 17 '16 at 19:38
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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.

Dharam Vir Ahluwalia
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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.

Lightness Races in Orbit
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