Sterile neutrino dark matter catalyzed by a very light dark photon

Sterile neutrinos ($\nu_s$) that mix with active neutrinos ($\nu_a$) are interesting dark matter candidates with a rich cosmological and astrophysical phenomenology. In their simplest incarnation, their production is severely constrained by a combination of structure formation observations and X-ray searches. We show that if active neutrinos couple to an oscillating condensate of a very light $L_{\mu}-L_{\tau}$ gauge field, resonant $\nu_a$-$\nu_s$ oscillations can occur in the early universe, consistent with $\nu_s$ constituting all of the dark matter, while respecting X-ray constraints on $\nu_s\to\nu_a\gamma$ decays. Interesting deviations from standard solar and atmospheric neutrino oscillations can persist to the present.