  Ultra-light hidden-photon dark matter produces an oscillating electric field
in the early Universe plasma, which in turn induces an electric current in its
ionized component whose dissipation results in heat transfer from the dark
matter to the plasma. This will affect the global 21cm signal from the Dark
Ages and Cosmic Dawn. In this work we focus on the latter, in light of the
reported detection by the EDGES collaboration of an absorption signal at
frequencies corresponding to redshift z~17. By measuring the 21cm global
signal, a limit can be placed on the amount of gas heating, and thus the
kinetic mixing strength $\varepsilon$ between the hidden and ordinary photons
can be constrained. Our inferred 21cm bounds on $\varepsilon$ in the mass range
$10^{-23}\,{\rm eV}\lesssim m_\chi\lesssim10^{-13}\,{\rm eV}$ are the strongest
to date.
