[tjradcliffe]

> It seems given the sheer amount of it, that the universe should be taking a radiation bath from the past history of insane amounts of dark matter anihilating.

The industry of dark matter modelling in cosmology involves attempting to find a set of characteristics of candidate particles that ensures we a) have enough dark matter on the right scales to account for the observed motions of distant galaxies (exotic dark matter is not required to explain the rotation curves of spiral galaxies, as there may be enough normal matter to do that particular job) and b) the dark matter has characteristics that don't contradict any current observations. Bonus points for a model if it makes some clear predictions of things that have not yet been observed.

All of this is intensely mathematical, because as it turns out the human intestine is a lousy judge of physical phenomena. Our guts believe in Aristotle's physics, and give us wildly wrong intuitions about the way the world is. They do a bad enough job with earthly phenomena, much less cosmological ones.

It turns out that the math tells us it is possible to have weakly interacting massive particles that simply don't decay into anything interesting on the timescale of the life of the universe, and don't interact with normal matter strongly enough to be much slowed down by the Earth. A WIMP in the halo of the galaxy, though, is going to pass through entire stars now and then, as well as large clouds of dust and gas that may not be very dense, but which have enormous thickness. As such these weakly interacting particles can be drawn in to ordinary matter on very large scales.

These mathematical constraints are vital to narrowing down what dark matter consists of (if it is a particle at all, and not a euphemism for some novel dynamical effect... there was a brief time a few years ago when someone claimed that we were solving the field equations of General Relativity in a subtly wrong way that explained the galactic observations, but they didn't turn out to have a compelling argument.)