Astronomers from the Center for Astrophysics | Harvard & Smithsonian (CfA) and Caltech have released groundbreaking research in Nature Astronomy that pinpoints the location of the Universe’s “missing” matter and detects the most distant fast radio burst (FRB) on record.
A landmark study led by the Center for Astrophysics | Harvard & Smithsonian (CfA) has pinpointed the Universe’s “missing” matter using Fast Radio Bursts (FRBs)— brief, bright radio signals from distant galaxies— as a guide. This artist’s conception depicts a bright pulse of radio waves (the FRB) on its journey through the fog between galaxies, known as the intergalactic medium. Long wavelengths, shown in red, are slowed down compared to shorter, bluer wavelengths, allowing astronomers to “weigh” the otherwise invisible ordinary matter. Image Credit: Melissa Weiss/CfA
Using FRBs as a guide, astronomers discovered that more than three-quarters of the Universe's ordinary matter is hidden in the thin gas between galaxies, representing a significant advance in understanding how matter interacts and functions in the Universe. They utilized the new data to conduct the first thorough measurement of the distribution of ordinary matter throughout the cosmic web.
For decades, scientists knew that at least half of the Universe’s ordinary (or baryonic) matter, which is mostly made up of protons, was unaccounted for.
Previously, scientists used methods like X-ray emission and ultraviolet scans of distant quasars to search for massive amounts of missing matter, believed to exist as an extremely thin, heated gas between galaxies. Because this gas is both hot and low in density, it remains mostly invisible to telescopes, making it possible to estimate its presence, but not directly confirm its quantity or exact location.
Then come FRBs, which are short, bright radio signals from galaxies far away. The 60 FRBs examined in the current study ranged in distance from around 11.74 million light years away (FRB20200120E in galaxy M81) to approximately 9.1 billion light years away (FRB 20230521B, the most distant FRB ever recorded). This enabled them to identify the missing matter as the intergalactic medium (IGM), the region between galaxies.
The decades-old 'missing baryon problem' was never about whether the matter existed. It was always: Where is it? Now, thanks to FRBs, we know: three-quarters of it is floating between galaxies in the cosmic web.
Liam Connor, Study Lead Author and Astronomer, Center for Astrophysics | Harvard & Smithsonian (CfA)
To put it another way, scientists now know the “missing” matter’s home address.
Connor and his colleagues followed the gas as it traveled across space by measuring how much each FRB signal was slowed down.
Connor added, “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.”
The results were clear: about 76% of the Universe’s baryonic matter is contained in the IGM. A tiny percentage is buried in stars or in cold galactic gas, while 15% live in galaxy halos.
Although this distribution has never been directly verified, it is consistent with predictions from sophisticated cosmological models.
It's a triumph of modern astronomy. 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.
Vikram Ravi, Study Co-Author and Assistant Professor, California Institute of Technology
It takes more than just creating an address book or conducting a census to locate the missing baryons. The key to solving profound riddles concerning the formation of galaxies, the clumping of matter in the Universe, and the motion of light across billions of light-years lies in their distribution.
“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. Our results show this feedback must be efficient, blasting gas out of galaxies and into the IGM,” stated Connor.
And FRB cosmology is only getting started.
“We're entering a golden age. 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,” concluded Ravi, who also serves as the co-PI of Caltech’s Deep Synoptic Array-110 (DSA-110)
Journal Reference:
Connor, L., et al. (2025) A gas-rich cosmic web revealed by the partitioning of the missing baryons. Nature Astronomy. doi.org/10.1038/s41550-025-02566-y