Astronomers have used Fast Radio Bursts (FRBs) to show that more than three-quarters of the Universe’s ordinary matter has been hiding in the thin gas between galaxies, marking a major step forward in understanding how matter interacts and behaves in the Universe. They’ve also detected the most distant FRB event on record.
This artist’s conception shows denser regions of the cosmic web in blue and void areas in red. Image crdit: Jack Madden / IllustrisTNG / Ralf Konietzka / Liam Connor, CfA.
For decades, scientists have known that at least half of the Universe’s ordinary, or baryonic matter — composed primarily of protons — was unaccounted for.
Previously, astronomers have used techniques including X-ray emission and ultraviolet observations of distant quasars to find hints of vast amounts of this missing mass in the form of very thin, warm gas in between galaxies.
Because that matter exists as hot, low-density gas, it was largely invisible to most telescopes, leaving scientists to estimate but not confirm its amount or location.
Enter FRBs — brief, bright radio signals from distant galaxies that scientists only recently showed could measure baryonic matter in the Universe, but until now could not find its location.
In the new study, researchers analyzed 60 FRBs, ranging from 11.74 million light-years away (FRB 20200120E) in Messier 81 to 9.1 billion light-years away (FRB 20230521B), the most distant FRB on record.
This allowed them to pin down the missing matter to the space between galaxies, or the intergalactic medium (IGM).
“The decades-old ‘missing baryon problem’ was never about whether the matter existed,” said Dr. Liam Connor, an astronomer at the Harvard & Smithsonian’s Center for Astrophysics.
“It was always: Where is it? Now, thanks to FRBs, we know: three-quarters of it is floating between galaxies in the cosmic web.”
By measuring how much each FRB signal was slowed down as it passed through space, Dr. Connor and colleagues tracked the gas along its journey.
“FRBs act as cosmic flashlights. They shine through the fog of the intergalactic medium, and by precisely measuring how the light slows down, we can weigh that fog, even when it’s too faint to see,” Dr. Connor said.
The results were clear: approximately 76% of the Universe’s baryonic matter lies in the IGM.
About 15% resides in galaxy halos, and a small fraction is burrowed in stars or amid cold galactic gas.
This distribution lines up with predictions from advanced cosmological simulations, but has never been directly confirmed until now.
“It’s a triumph of modern astronomy,” said Dr. Vikram Ravi, an astronomer at Caltech.
“We’re beginning to see the Universe’s structure and composition in a whole new light, thanks to FRBs.”
“These brief flashes allow us to trace the otherwise invisible matter that fills the vast spaces between galaxies.”
“Baryons are pulled into galaxies by gravity, but supermassive black holes and exploding stars can blow them back out — like a cosmic thermostat cooling things down if the temperature gets too high,” Dr. Connor said.
“Our results show this feedback must be efficient, blasting gas out of galaxies and into the IGM.”
The team’s results appear today in the journal Nature Astronomy.
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L. Connor et al. A gas-rich cosmic web revealed by the partitioning of the missing baryons. Nat Astron, published online June 16, 2025; doi: 10.1038/s41550-025-02566-y
