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Astronomers Just Solved the Mystery of the Universe's Missing Matter

A new study reveals that 76% of all ordinary matter exists in the
form of hot intergalactic gas.
Margherita Bassi
By Margherita Bassi Published June 16, 2025 | Comments (41) |
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Artistic rendering of an FRB. Artistic rendering of an FRB. (c) Melissa
Weiss/CfA

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?

Astronomyelectromagnetic wavesMatter

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