  Observations of the redshifted 21-cm signal (in absorption or emission) allow
us to peek into the epoch of "dark ages" and the onset of reionization. These
data can provide a novel way to learn about the nature of dark matter, in
particular about the formation of small size dark matter halos. However, the
connection between the formation of structures and 21-cm signal requires
knowledge of stellar to total mass relation, escape fraction of UV photons, and
other parameters that describe star formation and radiation at early times.
This baryonic physics depends on the properties of dark matter and in
particular in warm-dark-matter (WDM) models, star formation may follow a
completely different scenario, as compared to the cold-dark-matter case. We use
the recent measurements by the EDGES [J. D. Bowman, A. E. E. Rogers, R. A.
Monsalve, T. J. Mozdzen, and N. Mahesh, An absorption profile centred at 78
megahertz in thesky-averaged spectrum,Nature (London) 555, 67 (2018).] to
demonstrate that when taking the above considerations into account, the robust
WDM bounds are in fact weaker than those given by the Lyman-$\alpha$ forest
method and other structure formation bounds. In particular, we show that
resonantly produced 7 keV sterile neutrino dark matter model is consistent with
these data. However, a holistic approach to modelling of the WDM universe holds
great potential and may in the future make 21-cm data our main tool to learn
about dark matter clustering properties.
