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A new landmark study has pinpointed the location of the universe's "missing" matter, and detected the most distant fast radio burst (FRB) on record. Using FRBs as a guide, astronomers at the Center for Astrophysics | Harvard & Smithsonian (CfA) and Caltech have shown 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 used the new data to make the first detailed measurement of ordinary matter distribution across the cosmic web. The research is in the journal Nature Astronomy.

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—FRB20200120E in galaxy M81—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 Liam Connor, CfA astronomer and lead author of the new study. "It was always: Where is it? Now, thanks to FRBs, we know: three-quarters of it is floating between galaxies in the cosmic web." In other words, scientists now know the home address of the "missing" matter.

By measuring how much each FRB signal was slowed down as it passed through space, Connor and his team tracked the gas along its journey. "FRBs act as cosmic flashlights," Connor, who is also an assistant professor of astronomy at Harvard, said. "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."

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 Vikram Ravi, an assistant professor of astronomy at Caltech and co-author of the paper. "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."

Finding the missing baryons isn't just an exercise in building an address book or taking a census. Their distribution holds the key to unlocking deep mysteries about how galaxies form, how matter clumps in the universe, and how light travels across billions of light-years.

"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," said Connor. "Our results show this feedback must be efficient, blasting gas out of galaxies and into the IGM."

And this is just the beginning for FRB cosmology. "We're entering a golden age," said Ravi, who also serves as the co-PI of Caltech's Deep Synoptic Array-110 (DSA-110). "Next-generation radio telescopes like the DSA-2000 and the Canadian Hydrogen Observatory and Radio-transient Detector will detect thousands of FRBs, allowing us to map the cosmic web in incredible detail."