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As far as we know, the particles of dark matter can interact with each other only by gravitation. No electromagnetics, no weak force, no strong force. So, let's suppose a local slight concentration of dark matter comes about by chance motions and begins to gravitate. The particles would fall "inward" towards the center of the concentration. However, with no interaction to dissipate angular momentum, they would just orbit the center of the concentration and fly right back out to the vicinity of where they started resulting in no increase in density. Random motions would eventually wipe out the slight local concentration and we are left with a uniform distribution again.

How does dark matter form lumps?

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A friend in my astronomy club asked me this question, and after several tries, I was unable to answer it satisfactorily. –  Michael Luciuk Jan 27 '12 at 19:05
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No weak force is not known. –  dmckee Jan 27 '12 at 19:58
    
I do not understand why they would not with some probability as they gravitate have 0 angular momentum between two of them, and stick together into a particle with a higher mass. After all meteorites fall and stick on the earth, some comets fall on the moon etc. So even with only gravity they will cluster, imo. –  anna v Jan 27 '12 at 20:03
    
Also, half of the point is that dark matter is significantly less lumpy than ordinary matter--the dark matter is concentrated in spherically symmetric distributions out in the galactic halo, where it hasn't been allowed to fall into the center of the galaxy where the ordinary matter is. (but most of the candidates still interact weakly, not to mention that there could be some sort of fifth force, or all sorts of things, once you start considering everything) –  Jerry Schirmer Jan 27 '12 at 22:28
    
"the particles of dark matter can interact with each other only by gravitation" We can know that they interact with normal matter only by gravitation, but how could we know how they interact with each other? –  endolith Oct 31 '13 at 17:46

5 Answers 5

The prevailing theory of dark matter is the Cold Dark Matter (CDM) hypothesis. This hypothesis is favored because it is assumed that the dark matter particles are non-relativistic - i.e. slow moving. Because they are slow moving, they can essentially orbit in and around the original small density fluctuations, making these small density fluctuatuions stable. These small density fluctuations can clump into denser clumps due to three body gravitational interactions. A three body interaction of small clumps can result in one of the clumps being ejected at a higher speed while the other two clumps slow down and become more gravitationally bound.

However, baryonic matter can clump more effectively than CDM since the electromagnetic interractions allow baryonic matter to cool more effectively than the CDM clumps. That is why the prevailing theory is that the DM forms halos that are more distended than the clumps of baryonic matter. Thus a visible galaxy will have a more extended DM halo that extends far beyond the visible stars of the galaxy. The DM halo will also be more spherical than the flattened galactic disk.

It is true that most DM models assume the DM particles do have weak interactions, but these interactions aren't required by the CDM model. However, these weak interactions are required if any of the dark matter detection experiments are to be successful.

[Note: After more research, I discovered that my original answer was wrong. I now think this answer is correct. Sorry about that.]

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FrankH: Which DM candidates interreact via the weak force? –  Michael Luciuk Jan 27 '12 at 21:17
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one, hasn't the 3-body thing been shown to have meaning for a larger system? I remember this being used to demonstrate the ultimate collapsing of matter in the universe on the local group scale, should no universal expansion or contraction happen. Two, if they can't interact by any mechanism other than gravity, doesn't GR dictate that the relative gravitation/kinetic energy be dissipated by gravitational waves? That's a net energy outflow, so yes, they would clump by that mechanism at the cost of space-time getting wavy like a pool surface. –  Alan Rominger Jan 27 '12 at 21:25
    
All the dark matter candidates that are being searched for by CDMS, XENON100 and even LHC - none of these experiments could ever detect a dark matter particle that only had gravitational interactions. The most popular theoretical dark matter particle is called a WIMP for Weakly Interacting Massive Particle and if supersymetry is correct, the least massive supersymmetric particle would be a WIMP and could be the dark matter particle. –  FrankH Jan 27 '12 at 21:31
    
FrankH: The weak force interaction would be the analogue of electromagnetic interaction in baryons to induce particle condensation. Can you describe the process, and suggest a reference on weak force DM interactions? –  Michael Luciuk Jan 27 '12 at 22:33
    
I did some looking on the web and appears I am wrong. I have updated my answer to what I now believe is correct. –  FrankH Jan 27 '12 at 23:30

In fact, three and more body purely gravitational interactions can form clusters and clumps by concentrating some particles and expelling others, as mentioned in a previous answer. Astronomers have discovered that this seems to happen quite rapidly. They call it "violent relaxation". Google "violent relaxation" for more info.

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Many (most?) of the candidates for dark matter interact via the weak force. If they didn't interact with anything then as you say they could not form clumps.

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Cold dark matter is usually assumed to interact only under gravity and computing simulations are done with N particles to see how they would arrange themselves on the scales of galaxies, or globular clusters. In order to clump under gravity, the dark matter particles need to lose angularly momentum, this is done from 3 body interactions, where a pair of particles 'kicks' another particle to have a higher angular momentum while the pair lose some of their angular momentum and get closer in orbit to the center of mass. Dark Matter researchers then find the profile of the dark matter from the simulations, usually finding a clusp like center (1/r)^n density profile (n<-1, i've seen n=1.7 often, but this varies with the detail of the simulations). Dark Matter (especially the supersymmetric LSP type), also might self annilihate, from point-like weak forces when two particle overlap, this is a rare process but which leads to observable high energy gamma rays, which would have a (1/r)^2n like profile. Dark Matter researchers are currently divided on weather such gamma rays have been observered (e.g. by the FERMI and HESS satellites), and also worry weither the dark matter HALO around galaxies are well described by (1/r)^n scale profiles. If researchers don't see something like a (1/r)^-2.5 gamma ray patterns from the galactic center or something close, they may need to find alternative models for dark matter.

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My problem is with the time scale of the phenomenon. It has been proposed that the universal network of filaments and concentrated lumps of dark matter is the framework upon which ordinary matter condensed to form our present day clusters of galaxies. If this is so, it would seem reasonable that there be some rapid mechanism for its formation early in the development of the universe. With only gravity to draw it together and mechanisms like the three body interaction expelling one of the bodies to permit coalescence into higher density lumps, how did the dark matter network manage to form first? Ordinary matter has the same gravitational means of aggregating plus the electromagnetic interactions for reducing angular momentum. Why didn't the ordinary matter condense first?

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