  Sterile neutrinos with masses in the keV range can be the dark matter, and
their emission from a supernova can explain the observed velocities of pulsars.
The sterile neutrino decays could produce the x-ray radiation in the early
universe, which could have an important effect on the formation of the first
stars. X-rays could ionize gas and could catalyze the production of molecular
hydrogen during the ``dark ages''. The increased fraction of molecular hydrogen
could facilitate the cooling and collapse of the primordial gas clouds in which
the first stars were formed.
