  Sterile neutrinos with keV masses can constitute all or part of the
cosmological dark matter. The electroweak-singlet fermions, which are usually
introduced to explain the masses of active neutrinos, need not be heavier than
the electroweak scale; if one of them has a keV-scale mass, it can be the
dark-matter particle, and it can also explain the observed pulsar kicks. The
relic sterile neutrinos could be produced by several different mechanisms. If
they originate primarily from the Higgs decays at temperatures of the order of
100 GeV, the resulting dark matter is much ``colder'' than the warm dark matter
produced in neutrino oscillations. The signature of this form of dark matter is
the spectral line from the two-body decay, which can be detected by the X-ray
telescopes. The same X-rays can have other observable manifestations, in
particular, though their effects on the formation of the first stars.
