  The recent EDGES collaboration detection of an absorption signal at a central
frequency of $\nu = 78 \pm 1$ MHz points to the presence of a significant
Lyman-$\alpha$ background by a redshift of $z=18$. The timing of this signal
constrains the dark matter particle mass ($m_\chi$) in the warm dark matter
(WDM) cosmological model. WDM delays the formation of small-scale structures,
and therefore a stringent lower limit can be placed on $m_\chi,$ based on the
presence of a sufficiently strong Ly-$\alpha$ background due to star formation
at $z=18$. Our results show that the coupling the spin temperature to the gas
through Ly-$\alpha$ pumping requires a minimum mass of $m_\chi>3$ keV if atomic
cooling halos dominate the star formation rate at $z=18,$ and $m_\chi>2$ keV if
${\rm\,H_2}$ cooling halos also form stars efficiently at this redshift. These
limits match or exceed the most stringent limits cited to date in the
literature, even in the face of the many uncertainties regarding star-formation
at high redshift.
