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.

Reliable values of quark and lepton masses are important for model building at a fundamental energy scale, such as the Fermi scale $M_Z\approx 91.2\mathrm{GeV}$ and the would-be GUT scale ${\Lambda }_{\mathrm{GUT}}\sim 2\times {10}^{16}\mathrm{GeV}$. Using the latest data given by the Particle Data Group, we update the running quark and charged-lepton masses at a number of interesting energy scales below and above $M_Z$. In particular, we take into account the possible new physics scale ($\mu \sim 1\mathrm{TeV}$) to be explored by the CERN LHC and the typical seesaw scales ($\mu \sim {10}^9\mathrm{GeV}$ and $\mu \sim {10}^{12}\mathrm{GeV}$) which might be relevant to the generation of neutrino masses. For illustration, the running masses of three light Majorana neutrinos are also calculated. Our up-to-date tables of running fermion masses are expected to be very useful for the study of flavor dynamics at various energy scales.

We investigate possible interactions between neutrinos and massive scalar bosons via $g_{\varphi }\overline{\nu }\nu \varphi$ (or massive vector bosons via $g_V\overline{\nu }{\gamma }^{\mu }\nu V_{\mu }$) and explore the allowed parameter space of the coupling constant $g_{\varphi }$ (or $g_V$) and the scalar (or vector) boson mass $m_{\varphi }$ (or $m_V$) by requiring that these secret neutrino interactions (SNIs) should not spoil the success of big bang nucleosynthesis (BBN). Incorporating the SNIs into the evolution of the early Universe in the BBN era, we numerically solve the Boltzmann equations and compare the predictions for the abundances of light elements with observations. It turns out that the constraint on $g_{\varphi }$ and $m_{\varphi }$ in the scalar-boson case is rather weak, due to a small number of degrees of freedom (d.o.f.). However, in the vector-boson case, the most stringent bound on the coupling $g_V\lesssim 6\times {10}^{-10}$ at 95% confidence level is obtained for $m_V\simeq 1\mathrm{MeV}$, while the bound becomes much weaker $g_V\lesssim 8\times {10}^{-6}$ for smaller masses $m_V\lesssim {10}^{-4}\mathrm{MeV}$. Moreover, we discuss in some detail how the SNIs affect the cosmological evolution and the abundances of the lightest elements.