A new life for sterile neutrino dark matter after the pandemic

We propose a novel mechanism to generate sterile neutrinos ${\nu }_s$ in the early Universe, by converting ordinary neutrinos ${\nu }_{\alpha }$ in scattering processes ${\nu }_s{\nu }_{\alpha }\to {\nu }_s{\nu }_s$. After initial production by oscillations, this leads to an exponential growth in the ${\nu }_s$ abundance. We show that such a production regime naturally occurs for self-interacting ${\nu }_s$, and that this opens up significant new parameter space where ${\nu }_s$ make up all of the observed dark matter. Our results provide strong motivation to further push the sensitivity of X-ray line searches, and to improve on constraints from structure formation.