Astronomers believe that a large part of the mass in the universe is "dark." But what is the nature of this unseen mass? In his research commentary, Primack discusses results reported in the same issue by Gawiser and Silk in which several models of dark matter were examined. The one that best agrees with the data is a model in which the universe is made of 70% cold dark matter, 20% hot dark matter, and 10% ordinary matter. Primack discusses in particular the nature of the hot dark matter, which may consist of neutrinos with mass.