  We present a comprehensive review of keV-scale sterile neutrino Dark Matter,
collecting views and insights from all disciplines involved - cosmology,
astrophysics, nuclear, and particle physics - in each case viewed from both
theoretical and experimental/observational perspectives. After reviewing the
role of active neutrinos in particle physics, astrophysics, and cosmology, we
focus on sterile neutrinos in the context of the Dark Matter puzzle. Here, we
first review the physics motivation for sterile neutrino Dark Matter, based on
challenges and tensions in purely cold Dark Matter scenarios. We then round out
the discussion by critically summarizing all known constraints on sterile
neutrino Dark Matter arising from astrophysical observations, laboratory
experiments, and theoretical considerations. In this context, we provide a
balanced discourse on the possibly positive signal from X-ray observations.
Another focus of the paper concerns the construction of particle physics
models, aiming to explain how sterile neutrinos of keV-scale masses could arise
in concrete settings beyond the Standard Model of elementary particle physics.
The paper ends with an extensive review of current and future astrophysical and
laboratory searches, highlighting new ideas and their experimental challenges,
as well as future perspectives for the discovery of sterile neutrinos.
