  We explore the range of parameters for dark-matter sterile neutrinos in an
extention of the Minimal Standard Model by three singlet fermions with masses
below the electroweak scale (the $\nu$MSM). This simple model can explain a
wide range of phenomena, including neutrino oscillations, baryogenesis, the
pulsar velocities, and the early reionization. The presence of two heavier
sterile neutrinos and the possibility of entropy production in their decays
broadens the allowed range of parameters for the dark-matter sterile neutrinos
(or other types of dark matter, for example, the gravitino). In addition, the
primordial production of dark matter sterile neutrinos allows to escape most of
the constraints.
