Hidden in the neutrino sector may be one or more fermions with no
standard model interactions that nonetheless couple to neutrinos via
their mass generation mechanism, namely sterile neutrinos. Such a
particle may be the dark matter, produced in the early universe
through matter-suppressed neutrino mixing or matter-enhanced resonant
mixing. I will overview the kinetics of relativistic mixed neutrinos
in dense environments, and will specify with sterile neutrino dark
matter production in the early universe. I will discuss how this
candidate alters cosmological structure formation and the resulting
constraints from observed cosmological clustering. In addition, I
discuss how this candidate may be detected by X-ray telescopes.