For decades, scientists have been searching for something strange: a huge portion of the universe’s ordinary matter seemed to be missing.
Not dark matter, but the everyday kind made of protons and neutrons—called baryonic matter—that makes up stars, planets, and people.
While theories predicted where it should be, astronomers simply couldn’t find about half of it. Now, thanks to brief flashes of radio light from deep space, researchers have finally solved the mystery.
In a major new study, scientists from the Center for Astrophysics | Harvard & Smithsonian and Caltech have pinpointed where most of the missing matter is hiding.
It turns out, more than three-quarters of the universe’s ordinary matter is floating in the thin, hot gas between galaxies, known as the intergalactic medium (IGM).
This finding gives astronomers a much clearer picture of how the universe is structured and how matter behaves over cosmic time.
The key to this discovery lies in fast radio bursts, or FRBs. These are extremely short but powerful flashes of radio waves that come from faraway galaxies. They were only discovered recently, and their exact causes are still being studied.
But scientists realized that FRBs can serve as tools to probe the matter between galaxies. As each FRB travels through space, its signal slows slightly depending on how much gas it passes through. By carefully measuring this slowdown, astronomers can track the amount of matter along the burst’s path.
In the new study, the team analyzed 60 FRBs, including one from a galaxy just over 11 million light-years away and another from nearly 9.1 billion light-years away—the most distant FRB ever recorded. Using this data, they were able to weigh the gas spread throughout the cosmos and map where the missing matter actually is.
Lead author Liam Connor, an astronomer at the CfA and Harvard, explained that the problem was never whether the missing matter existed—it was always about where it had gone.
Thanks to FRBs, we now know it’s largely floating in the vast, thin “fog” of the intergalactic medium. These bursts act like cosmic flashlights, lighting up what was previously invisible to telescopes.
The results show that about 76% of the universe’s baryonic matter lies in the IGM. Another 15% sits in the halos surrounding galaxies, and only a small amount is locked up in stars or cold gas inside galaxies. These results match what computer models had predicted, but this is the first time scientists have directly confirmed it.
Understanding where this matter is helps scientists learn how galaxies form and evolve. As Connor put it, “Galaxies pull matter in with gravity, but powerful events like exploding stars and black holes can blow it back out, like a cosmic thermostat.” This movement of gas plays a huge role in shaping the universe.
This discovery is just the beginning. With more powerful radio telescopes coming soon, such as Caltech’s DSA-2000 and Canada’s CHORD, researchers expect to find thousands more FRBs.
These signals will allow scientists to trace the universe’s hidden structures in even greater detail, marking the dawn of a new era in understanding the cosmos.
