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.

Neutrinos, and in particular their tiny but non-vanishing masses, can be considered one of the doors towards physics beyond the Standard Model. Precision measurements of the kinematics of weak...

We present a measurement of the low-energy (0–60 keV) $\gamma$-ray spectrum produced in the $\alpha$ decay of ${}^{233}\mathrm{U}$ using a dedicated cryogenic magnetic microcalorimeter. The energy resolution of $\sim 10\mathrm{eV}$, together with exceptional gain linearity, allows us to determine the energy of the low-lying isomeric state in ${}^{229}\mathrm{Th}$ using four complementary evaluation schemes. The most precise scheme determines the ${}^{229}\mathrm{Th}$ isomer energy to be 8.10(17) eV, corresponding to 153.1(32) nm, superseding in precision previous values based on $\gamma$ spectroscopy, and agreeing with a recent measurement based on internal conversion electrons. We also measure branching ratios of the relevant excited states to be $b_{29}=9.3(6)\%$ and $b_{42}<0.7\%$.

This paper reports on the development of a technology involving $$^{100}\hbox