New season of "Universe's missing matter found"

Space

Technological Innovation Website Editorial Team - 06/23/2025

This artist's impression (not to scale) illustrates the path of a fast radio burst from the distant galaxy where it originated to Earth. [Image: ESO/M. Kornmesser]

Missing Matter of the Universe

The subject of the "missing matter of the Universe" is an old and somewhat embarrassing one for physicists and their theories. And the solutions to it have remained controversial over time.

First the problem: According to the standard cosmological model, 70% of the Universe is made up of dark energy (which we don't know what it is), 25% is dark matter (which we've never found) and 5% is ordinary matter, made up of atoms from the periodic table. The problem is that, so far, astronomers have only been able to detect 50% of this 5% of so-called baryonic matter, which has led to the coining of the term "missing matter of the Universe" - it's this list of "unknowns" that is embarrassing.

Some parts of the missing matter have been identified for years, but in 2016 a really interesting proposal emerged, thanks to the discovery of fast radio bursts (RRBs).

The cause of these flashes is still a mystery, but astrophysicists have realized that fast radio bursts can be used to weigh the universe and thus locate previously unidentified pieces of matter. Since these radio waves pass through all matter in their path, including cosmic dust, as they travel to Earth, viewing the galaxy and monitoring each FRB allows us to calculate the amount of matter present in the path.

The first study to use the fast bursts to detect the missing matter was contested because the burst used wasn't exactly what astronomers first proposed, but Liam Connor and colleagues at the Harvard & Smithsonian Center for Astrophysics left out the details of each particular FRB, opting instead to take a chance on the odds — using many of them.

Where is the missing matter of the Universe?

Rather than relying on a single FRB, the team captured no fewer than 60 of them, ranging from 11.74 million light-years away (FRB20200120E in the galaxy M81) to 9.1 billion light-years away (FRB 20230521B, the most distant yet recorded). This allowed them to calculate the baryonic matter present between galaxies, in the so-called intergalactic medium.

By measuring how much each FRB signal was slowed down as it passed through space, the team tracked the gas along its journey. “FRBs act like cosmic flashlights,” Connor explained. “They shine through the haze of the intergalactic medium, and by precisely measuring how the light slows down, we can assess that haze even when it is too faint to see.”

"The 'missing baryon problem,' which has been going on for decades, has never been about the existence of matter," Connor said. "It's always been: Where is it? Now, thanks to FRBs, we know: Three-quarters of it is floating between galaxies in the cosmic web."

The results are robust: Approximately 76% of the baryonic matter in the Universe is found in the intergalactic medium. About 15% is in the halos around galaxies, and a small fraction is buried in stars or in cold galactic gas. This distribution agrees with predictions from cosmological simulations, but has never been directly confirmed until now.

Points to tie

Finding the missing baryons is not just an exercise in building an address book or taking a census of matter. Their distribution holds the key to unlocking crucial phenomena - how galaxies form, how matter clumps together in the universe and how light travels across billions of light-years, for example.

"Baryons are pulled into galaxies by gravity, but supermassive black holes and exploding stars can expel them back out — like a cosmic thermostat cooling things down if the temperature gets too high," Connor said. "Our results show that this feedback loop must be efficient, expelling gas from galaxies into the intergalactic medium."

Astrophysicists are now continuing their search for more FRBs to increase the significance of their calculations. But there are still loose ends to be tied up: In addition to not knowing what causes these enormous bursts of energy, there is a relationship, known as the Macquart relation , which proposes that the more distant an FRB is found, the more diffuse gas it will reveal between galaxies - but not all FRBs obey this rule.

Other news about:

More topics