  Despite an intense theoretical and experimental effort over the past decade,
observations of the extragalactic radio background at multiple frequencies
below 10 GHz are not understood in terms of known radio sources, and may
represent a sign of new physics. In this Letter we identify a new class of dark
sector models with feebly interacting particles, where dark photons oscillate
into ordinary photons that contribute to the radio background. Our scenario can
explain both the magnitude and the spectral index of the radio background,
while being consistent with other cosmological and astrophysical constraints.
These models predict new relativistic degrees of freedom and spectral
distortions of the cosmic microwave background, which could be detected in the
next generation of experiments.
