  The cosmic radio-frequency spectrum is expected to show a strong absorption
signal corresponding to the 21-centimetre-wavelength transition of atomic
hydrogen around redshift 20, which arises from Lyman-alpha radiation from some
of the earliest stars. By observing this 21-centimetre signal - either its
sky-averaged spectrum or maps of its fluctuations, obtained using radio
interferometers - we can obtain information about cosmic dawn, the era when the
first astrophysical sources of light were formed. The recent detection of the
global 21-centimetre spectrum reveals a stronger absorption than the maximum
predicted by existing models, at a confidence level of 3.8 standard deviations.
Here we report that this absorption can be explained by the combination of
radiation from the first stars and excess cooling of the cosmic gas induced by
its interaction with dark matter. Our analysis indicates that the spatial
fluctuations of the 21-centimetre signal at cosmic dawn could be an order of
magnitude larger than previously expected and that the dark-matter particle is
no heavier than several proton masses, well below the commonly predicted mass
of weakly interacting massive particles. Our analysis also confirms that dark
matter is highly non-relativistic and at least moderately cold, and primordial
velocities predicted by models of warm dark matter are potentially detectable.
These results indicate that 21-centimetre cosmology can be used as a
dark-matter probe.
