Neutrino self-interaction parameters that can populate sterile neutrinos with the entire DM relic abundance, for several combinations of m ₄ and ﹩{\sin }^{2}2\theta ﹩ (gray curves). The dark green hatched region shows the constraint from invisible Z width; light green shaded regions show experimental flavor-dependent constraints (Berryman et al. 2018, 2023; Esteban et al. 2021). Diagonal black lines divide this parameter space into production regimes corresponding to Cases A, B, and C.