CHIME Collaboration Sheds Light on the Universe’s Missing Matter

When astronomers tally up all the ordinary, or “baryonic,” matter we can see—stars, planets, gas in galaxies—they come up short. A sizeable fraction simply seems to have vanished, earning the nickname “missing matter.” Now a cross‑border team, including researchers from McGill University and the Massachusetts Institute of Technology, has found a clever way to track it down.

The trick hinges on fast radio bursts (FRBs), those brief, ultra‑bright millisecond flashes of radio waves that originate in distant, energetic corners of the universe. As an FRB’s signal races across billions of light‑years, it gets subtly stretched by every particle it encounters. By measuring that stretch—called dispersion—scientists can infer how much matter the burst has traversed.

In this study, the team combined the dispersion data from 2,870 FRBs recorded by the Canadian Hydrogen Intensity Mapping Experiment (CHIME) telescope near Penticton, B.C., with the precise three‑dimensional map of more than 30 million galaxies gathered by the Dark Energy Spectroscopic Instrument (DESI) in Tucson. By cross‑matching the locations of galaxies with the FRB signals, they could separate the contribution of matter bound up in galaxies from the contribution of tenuous gas floating around them.

The result was striking: a substantial amount of the “missing” baryons resides in diffuse clouds that envelope individual galaxies and even whole galaxy groups, extending roughly four million light‑years from the galactic centers. That is far farther than most computer simulations had predicted, suggesting that energetic processes—black‑hole jets, exploding stars and similar outflows—are more vigorous at flinging material into intergalactic space than previously thought.

“Fast radio bursts are amazingly effective probes of the cosmic web,” said Victoria Kaspi, professor of physics at McGill. Lead author Haochen Wang, a graduate student at MIT’s Kavli Institute, added that the method not only confirms the presence of the hidden gas but actually maps where it lives. “By charting this missing matter we get a clearer picture of how galaxies grow and interact with their surroundings,” he noted.

Because CHIME continues to detect dozens of FRBs each week, the technique will only get sharper with time. As more bursts are cataloged, astronomers expect to refine the map of diffuse matter and perhaps uncover new clues about the forces that shape the large‑scale structure of the universe.