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The cosmic radio-frequency spectrum is expected to show a strong absorptionsignal corresponding to the 21-centimetre-wavelength transition of atomichydrogen around redshift 20, which arises from Lyman-alpha radiation from someof the earliest stars. By observing this 21-centimetre signal - either itssky-averaged spectrum or maps of its fluctuations, obtained using radiointerferometers - we can obtain information about cosmic dawn, the era when thefirst astrophysical sources of light were formed. The recent detection of theglobal 21-centimetre spectrum reveals a stronger absorption than the maximumpredicted by existing models, at a confidence level of 3.8 standard deviations.Here we report that this absorption can be explained by the combination ofradiation from the first stars and excess cooling of the cosmic gas induced byits interaction with dark matter. Our analysis indicates that the spatialfluctuations of the 21-centimetre signal at cosmic dawn could be an order ofmagnitude larger than previously expected and that the dark-matter particle isno heavier than several proton masses, well below the commonly predicted massof weakly interacting massive particles. Our analysis also confirms that darkmatter is highly non-relativistic and at least moderately cold, and primordialvelocities predicted by models of warm dark matter are potentially detectable.These results indicate that 21-centimetre cosmology can be used as adark-matter probe.

Possible interaction between baryons and dark-matter particles revealed by the first stars