Sean Carroll in Preposterous Universe:
It has often been said, including by me, that one of the most intriguing aspects of dark matter is that provides us with the best current evidence for physics beyond the Core Theory (general relativity plus the Standard Model of particle physics). The basis of that claim is that we have good evidence from at least two fronts — Big Bang nucleosynthesis, and perturbations in the cosmic microwave background — that the total density of matter in the universe is much greater than the density of “ordinary” matter like we find in the Standard Model.
There is one important loophole to this idea. The Core Theory includes not only the Standard Model, but also gravity. Gravitons themselves can’t be the dark matter — they’re massless particles, moving at the speed of light, while we know from its effects on galaxies that dark matter is “cold” (moving slowly compared to light). But there are massive, slowly-moving objects that are made of “pure gravity,” namely black holes. Could black holes be the dark matter?
It depends. The constraints from nucleosynthesis, for example, imply that the dark matter was not made of ordinary particles by the time the universe was a minute old. So you can’t have a universe with just regular matter and then form black-hole-dark-matter in the conventional ways (like collapsing stars) at late times. What you can do is imagine that the black holes were there from almost the start — that they’re primordial. Having primordial black holes isn’t the most natural thing in the world, but there are ways to make it happen, such as having very strong density perturbations at relatively small length scales (as opposed to the very weak density perturbations we see at universe-sized scales).
Recently, of course, black holes were in the news, when LIGO detected gravitational waves from the inspiral of two black holes of approximately 30 solar masses each. This raises an interesting question, at least if you’re clever enough to put the pieces together: could the dark matter be made of primordial black holes of around 30 solar masses, and could two of them have come together to produce the LIGO signal?