Detecting boosted dark photons with gaseous detectors

We search for indirect signals of $O\left(\mathrm{keV}\right)$ dark matter annihilating or decaying into $O\left(\mathrm{eV}\right)$ dark photons. These dark photons will be highly boosted, predominantly transversely polarized, have decay lengths larger than the Milky Way, and can be absorbed by neutrino or dark matter experiments at a rate dependent on the photon-dark photon kinetic mixing parameter and the optical properties of the experiment. We show that current experiments cannot probe new parameter space, but future large-scale gaseous detectors with low backgrounds (i.e., CYGNUS, NEXT, PANDAX-III) may be sensitive to this signal when the annihilation cross section is especially large.