A study led by the Harvard & Smithsonian Center for Astrophysics reveals that more than 75% of the baryonic matter in the cosmos is floating between galaxies, solving a decades-old mystery about the distribution of the universe’s visible material.

For decades, one of cosmology’s most persistent mysteries has been the so-called missing baryon problem: theoretical models and observations of the early universe suggested a specific amount of ordinary matter—made up mainly of protons and neutrons—but when observing the present-day cosmos, astronomers were only able to locate about half of that matter.

Now, a team of researchers from the Harvard & Smithsonian Center for Astrophysics (CfA), in collaboration with the California Institute of Technology (Caltech), has managed to locate this “missing material” thanks to a new cosmic tool: fast radio bursts, or FRBs.

FRBs are intense and extremely brief flashes of radio waves that originate from distant galaxies and, although they were discovered just over a decade ago, have already begun to revolutionize how dispersed matter in the universe is studied.

lost matter universe located
This artist’s conception depicts a bright pulse of radio waves (the FRB) on its journey through the fog between galaxies, known as the intergalactic medium. Long wavelengths, shown in red, are slowed down compared to shorter, bluer wavelengths, allowing astronomers to “weigh” the otherwise invisible ordinary matter. Credit: Melissa Weiss / CfA

In this research, published in the journal Nature Astronomy, scientists analyzed 60 of these pulses, covering distances ranging from 11.7 million to over 9.1 billion light-years, allowing them to map the matter found in the intergalactic medium—the space between galaxies that forms a kind of invisible cosmic fog to most conventional telescopes.

The question was never whether this matter existed, but where it was, explains Liam Connor, an astronomer at the CfA and lead author of the study. Thanks to FRBs, we now know that three-quarters of that ordinary matter is floating between galaxies, scattered throughout the cosmic web. In other words, for the first time, the scientific community has been able to pinpoint the “whereabouts” of that portion of matter that was known to exist, but previously untraceable.

The process that allows this localization is based on a relatively simple principle: as radio waves from an FRB pass through the intergalactic medium, they slow down slightly—and do so in proportion to the amount of matter they encounter along the way. Longer wavelengths (depicted in red in artistic illustrations of the phenomenon) are slowed more than shorter ones (blue), and this difference in speed allows astronomers to estimate the amount of matter they have passed through. Like cosmic flashlights, FRBs light the way and allow scientists to “weigh” the fog surrounding them, even if it cannot be seen directly.

The results obtained not only confirmed that approximately 76% of the universe’s baryonic matter is in the intergalactic medium, but also revealed that another 15% is in galaxy halos (regions surrounding these structures), and a smaller fraction is contained in stars or in cold gas clouds within galaxies themselves. This distribution matches the predictions made by advanced cosmological simulations, but until now it had not been directly confirmed.

lost matter universe located
A full account and partition of the missing baryons. Credit: L. Connor et al.

For Vikram Ravi, co-author of the study and professor at Caltech, this is a triumph of modern astronomy. According to him, FRBs have made it possible to observe the universe’s structure and composition from a new perspective, acting as a revolutionary tool to trace the matter that fills the vast empty spaces between galaxies.

Beyond resolving a quantitative mystery, knowing the precise location of baryonic matter advances the understanding of key processes in the universe, such as galaxy formation, the behavior of matter on large scales, or the propagation of light over billions of years. As Connor explains, these baryons are not static—gravity tends to draw them into galaxies, but energetic phenomena like supernova explosions or activity from supermassive black holes can eject them back into the intergalactic medium, functioning like a cosmic thermostat that regulates the universe’s thermal balance.

This work, however, does not represent an endpoint, but rather the beginning of a new phase in observational cosmology. Ravi notes that we are entering a “golden age” of studying the universe through FRBs, thanks to new telescopes like the DSA-2000 (Deep Synoptic Array) and CHORD (Canadian Hydrogen Observatory and Radio-transient Detector), which will allow the detection of thousands of these bursts and the mapping of the cosmic web in unprecedented detail.

With this research, published on June 17 in Nature Astronomy, the team has not only solved one of the oldest enigmas of the modern universe but also opened the door to a new era in which fleeting, invisible phenomena to the human eye reveal the deepest secrets of the cosmos.

SOURCES

Harvard-Smithsonian Center for Astrophysics

Connor, L., Ravi, V., Sharma, K. et al. A gas-rich cosmic web revealed by the partitioning of the missing baryons. Nat Astron (2025). doi.org/10.1038/s41550-025-02566-y

  • Share on:

Discover more from LBV Magazine English Edition

Subscribe to get the latest posts sent to your email.

Something went wrong. Please refresh the page and/or try again.

Follow LBV Magazine on Google News