I'm a mathematician, not a physicist, so I apologise in advance if I'm just showing my ignorance here. My only source of information on dark matter is popular science texts, like New Scientist. One thing that is never explained is how it gets to aggregate around galaxies. Lacking the ability to shed energy by radiation, it seems it should just fly straight past and through, never even getting trapped into an orbit. I can think of two possible explanations, neither very convincing. 1. Chaotic gravitational interactions catapulting much of it away. Just as planets can get ejected from solar systems, carrying off a lot of the system's KE, a large portion of the original dark matter flying through could leave with increased energy, allowing its kin to become trapped. If so, there must be constantly a great stream of dark matter coursing everywhere at greater than escape velocity. A problem with this is that it ought also to lead to normal matter constantly being flung out. 2. Thermodynamic cooling The normal matter in a galaxy, having shed energy as radiation, is thermodynamically cooler (i.e. less KE) than the dark matter zipping past it. That should lead to some transfer of energy through the gravitational interaction. A problem with this is that it would slow the cooling of the normal matter, delaying galaxy formation. As I understand it, it is already a challenge to explain how galaxies formed so swiftly.