Astronomers have successfully traced the bulk of the Universe's "missing" ordinary matter to the intergalactic medium, using fast radio bursts (FRBs) as cosmic probes. Researchers from the Center for Astrophysics | Harvard and Smithsonian and Caltech leveraged these powerful signals to detect the faint, elusive gas long suspected to float between galaxies, finally providing a detailed census of by Clarence Oxford
Los Angeles CA (SPX) Jun 17, 2025
Astronomers have successfully traced the bulk of the Universe's "missing" ordinary matter to the intergalactic medium, using fast radio bursts (FRBs) as cosmic probes. Researchers from the Center for Astrophysics | Harvard and Smithsonian and Caltech leveraged these powerful signals to detect the faint, elusive gas long suspected to float between galaxies, finally providing a detailed census of the Universe's baryonic content.
Although scientists have theorized that ordinary matter-composed mainly of protons-was hidden in hot, low-density gas between galaxies, previous efforts using X-rays and ultraviolet light from quasars only offered indirect evidence. This new research utilized a set of 60 FRBs, ranging from just over 11 million to more than 9 billion light-years away, to precisely measure how intergalactic gas slowed each signal.
"The decades-old 'missing baryon problem' was never about whether the matter existed," said lead author Liam Connor, an astronomer at the CfA. "It was always: Where is it? Now, thanks to FRBs, we know: three-quarters of it is floating between galaxies in the cosmic web."
FRBs enabled researchers to weigh this intergalactic "fog" by monitoring delays in signal arrival times, with Connor explaining, "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."
Findings indicate that about 76% of baryonic matter resides in the intergalactic medium, 15% in the halos of galaxies, and the remainder in stars or dense galactic gas. These proportions closely match theoretical models, marking the first time they have been directly confirmed.
"It's a triumph of modern astronomy," said Vikram Ravi, Caltech astronomer and study co-author. "We're beginning to see the Universe's structure and composition in a whole new light, thanks to FRBs."
The implications extend beyond cosmic bookkeeping. Understanding the distribution of baryons sheds light on galaxy formation and the mechanisms that regulate intergalactic matter flow. "Our results show this feedback must be efficient, blasting gas out of galaxies and into the IGM," Connor noted.
With next-generation radio telescopes such as the DSA-2000 and CHORD poised to detect thousands more FRBs, researchers are optimistic about mapping the cosmic web in unprecedented detail. "We're entering a golden age," said Ravi.
Research Report:A gas-rich cosmic web revealed by the partitioning of the missing baryons
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