  The discovery of neutrino masses suggests the likely existence of gauge
singlet fermions that participate in the neutrino mass generation via the
seesaw mechanism. The masses of the corresponding degrees of freedom can range
from well below the electroweak scale to the Planck scale. If some of the
singlet fermions are light, the sterile neutrinos appear in the low-energy
effective theory. They can play an important role in astrophysics and
cosmology. In particular, sterile neutrinos with masses of several keV can
account for cosmological dark matter, which can be relatively warm or cold,
depending on the production mechanism. The same particles can explain the
observed velocities of pulsars because of the anisotropy in their emission from
a cooling neutron star born in a supernova explosion. Decays of the relic
sterile neutrinos can produce a flux of X-rays that can affect the formation of
the first stars. Existing and future X-ray telescopes can be used to search for
the relic sterile neutrinos.
