  In the standard cosmology, it is believed that there are two relatively weak
and distinct band-limited absorption features, with the first absorption minima
near 20 MHz ($z\sim70$) and the other minima at higher frequencies between
50-110 MHz ($z\sim12-27$) in the global cosmological 21cm 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 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 21cm absorption 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 higher frequencies
where star formation and X-ray heating effects are expected to terminate the
absorption signal. In the standard cosmology and in excess gas cooling models
it is expected that 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 constraints, however future
searches for short range spin-dependent forces between electrons on the mm to
nm scale have the potential to discover the light mediator responsible for our
predicted signal.
