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.