  We discuss numerous mechanisms for production of sterile neutrinos, which can
account for all or a fraction of dark matter, and which can range from warm to
effectively cold dark matter, depending on the cosmological scenario. We
investigate production by Higgs boson decay, $(B-L)$ gauge boson production at
high temperature, as well as production via resonant and nonresonant neutrino
oscillations. We calculate the effects on structure formation in these models,
some for the first time. If two populations of sterile neutrinos, one warm and
one cold, were produced by different mechanisms, or if sterile neutrinos
account for only a fraction of dark matter, while the remainder is some other
cold dark matter particle, the resulting multi-component dark matter may
alleviate some problems in galaxy formation. We examine the X-ray constraints
and the candidate signal at 3.5 keV. Finally, we also show that the $\sigma_8$
problem can be a signature of fractional dark matter in the form of sterile
neutrinos in several mechanisms.
