Decades ago, astronomers estimated that “ordinary” matter (basically everything that isn’t dark matter or dark energy) makes up 5% of the universe. There was just one problem—they had no idea where most of it was.

Astronomers from the Center for Astrophysics Harvard & Smithsonian (CfA) and the California Institute of Technology (Caltech) have tracked down the universe’s “missing” matter. Fast radio bursts (FRBs) indicate that over three-quarters of ordinary matter, officially called baryonic matter, exists in the form of hot, low-density gas between galaxies. With this data, they claim to have documented the first detailed measurements of ordinary matter’s distribution throughout the universe.

“The decades-old ‘missing baryon problem’ was never about whether the matter existed,” CfA astronomer Liam Connor said in a CfA statement. “It was always: Where is it? Now, thanks to FRBs, we know: three-quarters of it is floating between galaxies in the cosmic web.” Connor and his colleagues explain their discovery in a study published today in Nature Astronomy.

Technically, baryonic matter is matter made of protons and neutrons. However, astronomers often use the term to refer to all matter made of atoms, which makes up everything that isn’t dark matter or dark energy. “A small fraction of baryons are in stars and the interstellar medium within galaxies,” the researchers wrote in the study. The interstellar medium is the space between star systems.

Previous research suggested that much of the remaining baryonic matter was gas spread throughout the intergalactic medium—the space between galaxies. But because “this diffuse ionized gas is notoriously difficult to measure,” the team added, scientists couldn’t confirm how much of the gas existed or exactly where it was.

In the new study, the researchers relied on FRBs—quick, bright radio signals from faraway galaxies. FRBs slow down when they travel through intergalactic gas. By measuring this decrease in speed, the team could infer how much gas the signal had traversed. They investigated 60 FRBs ranging from a galaxy 11.74 million light years away to one approximately 9.1 billion light years away. The latter emitted the most distant FRB known to science, designated FRB 20230521B.

By studying FRBs, scientists confirmed that around 76% of all baryonic matter exists in the IGM, 15% in galaxy halos, and another small fraction in stars or cold galactic gas. Cosmological simulations had previously suggested this distribution, but the recent study provides direct evidence, shedding light on the movement of matter across the universe.

“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, who is the lead author of the study. “Our results show this feedback must be efficient, blasting gas out of galaxies and into the IGM.”

“We’re beginning to see the Universe’s structure and composition in a whole new light, thanks to FRBs,” added Caltech astronomer and co-author Vikram Ravi. “These brief flashes allow us to trace the otherwise invisible matter that fills the vast spaces between galaxies.”

With increasingly powerful telescopes expected to detect thousands of FRBs, who knows what other mysteries might soon come to light?