Core-collapse supernovae stymie secret neutrino interactions

Beyond-the-Standard-Model interactions of neutrinos among themselves -- {\it secret interactions} in the supernova core may prevent shock revival, halting explosion. Besides, if en route to Earth undergo with relic neutrinos, neutrino burst reaching be down-scattered energy, falling below detection threshold. We probe through and apply our findings SN 1987A. place most stringent bounds on flavor-universal occurring a new mediator mass between 10 MeV 1 GeV.