  In the standard cosmology, it is believed that there are two weak and
distinct band-limited absorption features, near 20 MHz ($z\sim70$) and 90 MHz
($z\sim15$) in the global cosmological 21 cm signal which are signatures of
collisional gas dynamics in the cosmic dark ages and Lyman-$\alpha$ photons
from the first stars at cosmic dawn, respectively. A similar prediction of two
distinct band-limited, but stronger, absorption features is expected in models
with excess gas cooling, which have been invoked to explain the anomalous EDGES
signal. In this work, we explore a novel mechanism, where dark matter spin-flip
interactions with electrons through a light axial-vector mediator could
directly induce a 21 cm signal which is characteristically different from
either of these. We find generically, that our model predicts a strong,
broadband absorption signal extending from frequencies as low as 1.4 MHz
($z\sim1000$), from early in the cosmic dark ages where no conventional signal
is expected, all the way up to 90 MHz, depending upon the epoch of star
formation and X-ray heating. We find a rich set of spectral features that could
be probed in current and future experiments looking for the global 21 cm
signal. In standard cosmology and in excess gas cooling models the gas spin
temperature as inferred from the absorption signal is a tracer of the gas
kinetic temperature. However, in our model we find in certain regions of
parameter space that the spin temperature and kinetic temperature of the gas
evolve differently, and the absorption signal only measures the spin
temperature evolution. Large swathes of our model parameter space of interest
are safe from existing particle physics constraints, however future searches
for short range spin-dependent forces between electrons on the millimeter to
nanometer scale have the potential to discover the light mediator responsible
for our predicted signal.
