Re: Matter/antimatter vs "regular"/"dark" matter
As another poster has observed, gravity is an extremely weak force compared to the EM force experienced by both protons and electrons. Neutrons and neutrinos are neutral which is how they are able to wander through matter so easily, but neutrons feel the strong force and so collide with nuclei.
Hypothetical dark matter particles only feel the gravitational force, so collisions with one another will be elastic - meaning they don't lose overall kinetic energy (WIMPs of course will sometimes have weak interactions) . They are very unlikely to collide with light matter, just like neutrinos. Thus, the only mechanism for them to clump is the very small gravitational force, which means eventually they form halos around galaxies, but the halos will be very, very slow to contract.