That's a great question. Here's a first draft of a response, to be checked with experts. In order to settle into a cluster, a particle needs not only to be gravitationally attracted to it but also to have some way to dissipate energy. Think of a bouncing ball. If it weren't for various sorts of friction, it would just continue to bounce without ever settling down on the earth. If I understand correctly, the non-gravitational interactions of the hypothetical particles proposed to account for dark matter are just too weak to give enough dissipation to cause much clustering on scales smaller than a galaxy. A WIMP, for example, attracted to the earth might just pass right through it.
Mike W.
And our expert, Brian Fields, adds:
It also is worth noting that the dark matter represents most of the mass in a galaxy, so in practice galaxy formation is first characterized by dark matter forming the gravity potential wells, and baryons falling into these potentials. Then the dissipative nature of baryons (e.g.,
they can cool by radiating photons) allows them to be (much) more condensed than the large dark matter halos that surround visible galaxies.
A corollary to this is that if baryons didn't dissipate energy, they would have the same spatial distribution as the DM.
And finally, it also is true, as the questioner hints, that baryons can have gravitational effects on dark matter. In particular, the Sun should gravitationally capture the slowest DM particles, which should then annihilate in the Sun's center. Searches for such annihilation events are one of the ways we looks for dark matter.
Brian
(published on 02/20/2012)
