Hidden treasures: Sterile neutrinos as dark matter with miraculous abundance, structure formation for different production mechanisms, and a solution to the σ8 problem

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 multicomponent 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.