  After stars formed in the early Universe, their ultraviolet light is
expected, eventually, to have penetrated the primordial hydrogen gas and
altered the excitation state of its 21-centimetre hyperfine line. This
alteration would cause the gas to absorb photons from the cosmic microwave
background, producing a spectral distortion that should be observable today at
radio frequencies of less than 200 megahertz. Here we report the detection of a
flattened absorption profile in the sky-averaged radio spectrum, which is
centred at a frequency of 78 megahertz and has a best-fitting full-width at
half-maximum of 19 megahertz and an amplitude of 0.5 kelvin. The profile is
largely consistent with expectations for the 21-centimetre signal induced by
early stars, however, the best-fitting amplitude of the profile is more than a
factor of two greater than the largest predictions. This discrepancy suggests
that either the primordial gas was much colder than expected or the background
radiation temperature was hotter than expected. Astrophysical phenomena (such
as radiation from stars and stellar remnants) are unlikely to account for this
discrepancy, of the proposed extensions to the standard model of cosmology and
particle physics, only cooling of the gas as a result of interactions between
dark matter and baryons seems to explain the observed amplitude. The
low-frequency edge of the observed profile indicates that stars existed and had
produced a background of Lyman-alpha photons by 180 million years after the Big
Bang. The high-frequency edge indicates that the gas was heated to above the
radiation temperature less than 100 million years later.
