  Measurements of 21cm intensity mapping (IM) during the dark ages can
potentially provide us with an unprecedented window on high redshifts and small
scales. One of the main advantages this can bring involves the possibility to
probe the nature of dark matter. Tests of dark matter models with the
large-scale structure of the Universe are limited by non-linearities and
astrophysical effects, which are not present for IM measurements during the
dark ages. In this paper we focus on constraining the model in which dark
matter is comprised, totally or in part, by ultra-light axion-like particles
around the $10^{-18}-10^{-22}$ eV mass scale. For this model, the angular power
spectrum of 21cm brightness temperature fluctuations will exhibit a small-scale
suppression. However, this effect is intertwined with the imprint of
baryon-dark matter relative velocity at recombination, causing at the same time
an enhancement at large-scales, which is affected by the mass and abundance of
axion dark matter. In this work we forecast how future radio arrays will be
able to constrain ultra-light axion mass through both these effects on the
angular power spectrum.
