Astronomers have detected much of the Universe’s ordinary matter, which had long been missing from accounts of its total mass. Not ‘dark matter’ — the mysterious, invisible stuff that makes up the majority of the Universe’s contents. This is normal matter, but it’s spread so sparsely across intergalactic space that more than three-quarters of it is almost undetectable.

Using an array of 36 radio telescopes in remote Western Australia, researchers analysed the light from 6 fast radio bursts (FRBs), unusually energetic events that last just milliseconds and originate in other galaxies. The spectrum was sensitive enough to reveal the exceedingly thin matter that the FRBs met in their travels. “The missing matter was equivalent to only one or two atoms in a room the size of an average office,” says radio astronomer Jean-Pierre Macquart.

More than three-quarters of the baryonic content of the Universe resides in a highly diffuse state that is difficult to detect, with only a small fraction directly observed in galaxies and galaxy clusters. Censuses of the nearby Universe have used absorption line spectroscopy to observe the ‘invisible’ baryons, but these measurements rely on large and uncertain corrections and are insensitive to most of the Universe’s volume and probably most of its mass.

Universe’s invisible baryons

In particular, quasar spectroscopy is sensitive either to the very small amounts of hydrogen that exist in the atomic state, or to highly ionized and enriched gas in denser regions near galaxies. Other techniques to observe these invisible baryons also have limitations — Sunyaev–Zel’dovich analyses can provide evidence from gas within filamentary structures, and studies of X-ray emission are most sensitive to gas near galaxy clusters.

The scientists said a measurement of the baryon content of the Universe using the dispersion of a sample of localized fast radio bursts; this technique determines the electron column density along each line of sight and accounts for every ionized baryon.

“We augment the sample of reported arcsecond-localized fast radio bursts with four new localizations in host galaxies that have measured redshifts of 0.291, 0.118, 0.378 and 0.522. This completes a sample sufficiently large to account for dispersion variations along the lines of sight and in the host-galaxy environments, and we derive a cosmic baryon density of Ωb=0.051+0.021−0.025h−170 (95 per cent confidence; h70 = H0/(70 km s−1 Mpc−1) and H0 is Hubble’s constant,” wrote scientists in their paper published in Nature.

This independent measurement is consistent with values derived from the cosmic microwave background and from Big Bang nucleosynthesis, they wrote in their abstract.