2 (source code)
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For a quantitative thermal-relic prediction, assume one ordinary weakly interacting <neutrino> flavor: one populated <neutrino> helicity and one <antineutrino> helicity, negligible chemical potential, early thermal equilibrium, and relativistic decoupling around the MeV epoch. Equal <neutrino> and <antineutrino> populations imply no new lepton asymmetry. The <Fermi-Dirac distribution> is maintained while the <weak interaction> rate exceeds the expansion rate. After <neutrino decoupling>, the distribution free streams, its momentum scale falls as $a^{-1}$, and its comoving number is conserved. A mass of $1\,\mathrm{keV}$ is negligible at <Big Bang nucleosynthesis> temperatures, but the relic later becomes nonrelativistic. Its energy density then changes from radiation-like $a^{-4}$ scaling to matter-like $a^{-3}$ scaling. Stability prevents its removal by decay; decoupling while relativistic prevents a subsequent equilibrium Boltzmann suppression of its number.
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