  Sterile neutrinos are natural extensions to the standard model of particle
physics in neutrino mass generation mechanisms. If they are relatively light,
less than approximately 10 keV, they can alter cosmology significantly, from
the early Universe to the matter and radiation energy density today. Here, we
review the cosmological role such light sterile neutrinos can play from the
early Universe, including production of keV-scale sterile neutrinos as dark
matter candidates, and dynamics of light eV-scale sterile neutrinos during the
weakly-coupled active neutrino era. We review proposed signatures of light
sterile neutrinos in cosmic microwave background and large scale structure
data. We also discuss keV-scale sterile neutrino dark matter decay signatures
in X-ray observations, including recent candidate $\sim$3.5 keV X-ray line
detections consistent with the decay of a $\sim$7 keV sterile neutrino dark
matter particle.
