We propose a novel mechanism to generate sterile neutrinos
νs in the early Universe, by converting ordinary neutrinos
νa in scattering processes νsνa→νsνs.
After initial production by oscillations, this leads to an exponential
growth in the νs abundance. We show that such a production regime
naturally occurs for self-interacting νs, and that this opens up
significant new parameter space where νs make up all of the
observed dark matter.
