  We introduce a novel class of signatures---spectral edges and endpoints---in
21-cm measurements resulting from interactions between the standard and dark
sectors. Within the context of a kinetically mixed dark photon, we demonstrate
how resonant dark photon-to-photon conversions can imprint distinctive spectral
features in the observed 21-cm brightness temperature, with implications for
current, upcoming, and proposed experiments targeting the cosmic dawn and the
dark ages. These signatures open up a qualitatively new way to look for physics
beyond the Standard Model using 21-cm observations.
