  We apply phase-space density considerations to obtain lower bounds on the
mass of sterile neutrino as dark matter candidate. The bounds are different for
non-resonant production, resonant production in the presence of lepton
asymmetry and production in decays of heavier particles. In the former case our
bound is comparable to, but independent of the Lyman-alpha bound, and together
with X-ray upper limit it disfavors non-resonantly produced sterile neutrino
dark matter. An interesting feature of the latter case is that warm dark matter
may be composed of heavy particles.
