What is the dark matter in galactic halos and clusters?

Abstract

The dark matter in galactic halos and clusters could be very faint low-mass stars, or in black holes (the remnants of an early generation of very massive stars); alternatively, it could be some species of particle surviving from the early Universe. Although none of these three very different possibilities can yet be excluded, there are real prospects that observations and experiments may soon allow us to discriminate among them. The dynamically inferred dark matter contributes a fraction Ω = 0.1-0.2 of the critical cosmological density. The problem of reconciling the data with the theoretically appealing hypothesis that Ω = 1 is briefly addressed.