  The detection of massless kinetically-mixed dark photons is notoriously
difficult, as the effect of this mixing can be removed by a field redefinition
in vacuum. In this work, we study the prospect of detecting massless dark
photons in the presence of a cosmic relic directly charged under this dark
electromagnetism. Such millicharged particles, in the form of dark matter or
dark radiation, generate an effective dark photon mass that drives
photon-to-dark photon oscillations in the early universe. We also study the
prospect for such models to alleviate existing cosmological constraints on
massive dark photons, enlarging the motivation for direct tests of this
parameter space using precision terrestrial probes.
