A signal from the early Universe, 180 million years after the Big Bang, is reported in a study published in this week’s Nature. Although the signal is mostly as expected, a second Nature paper suggests that one property of it suggests that interactions between normal matter and dark matter took place within the early Universe.

An international effort is underway to detect signals from cosmic dawn - the period during which the first stars formed. Once stars formed in the early Universe, their ultraviolet light is predicted to have penetrated the primordial hydrogen gas, altering the excitation state of the electron in atoms of hydrogen. The transition is called the 21-cm hyperfine line and would cause the hydrogen gas to absorb photons from the cosmic microwave background, imprinting a signature in the radio spectrum that should be observable today at radio frequencies below 200 megahertz.

In the first paper, Judd Bowman and colleagues report the detection of such a signal at 78 megahertz. They find that the width of the observed profile is largely consistent with expectations, but that it has a larger amplitude (corresponding to deeper absorption) than predicted, indicating that the primordial gas was colder than expected.

In a separate study, Rennan Barkana suggests that the gas was cooled through the interaction of hydrogen with something that is even colder - dark matter. On the basis of the observed signal, the author argues that the dark-matter particle is no heavier than several proton masses.