This is the so-called non-baryonic matter that does not interact (or weakly interacts) with light and normal matter. The second problem is that most of this dark matter (maybe 5/6 of it) must be in a form that is not like the stuff that makes up the luminous stars, galaxies and you and me. The evidence for this includes the motions of stars and gas in galaxies, the motion of galaxies in clusters and the gravitational lensing of light by clusters of galaxies. One is that the amount of gravitating matter in the universe appears to be much larger (by a factor of roughly 30) than the amount of matter that we can actually see in the form of luminous stars and galaxies. There are actually two dark matter problems. The answer to the question "what is dark matter" is that nobody really knows.
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