Fast Radio Bursts (FRBs) have helped determine where the “missing” half of the universe’s expected matter resides, in new research from Caltech and the Center for Astrophysics scientists, with implications for future research into neutrinos.
Matter only makes up 16% of the universe, vastly eclipsed by invisible antimatter, which is known only through its gravitational effects. Yet, until now, scientists have only accounted for half of the matter that should exist in the universe, a long-standing cosmic mystery.
Fast Radio Bursts Light the Way
Designed to capture and identify the source of FRBs, Caltech’s DSA (Deep Synoptic Array)-110, a network of 110 radio telescopes funded by the National Science Foundation (NSF), sourced 39 of the bursts used in the study, with their distances measured by instruments at the Palomar Observatory outside of San Diego and the W. M. Keck Observatory in Hawaii.
Australia’s Square Kilometre Array Pathfinder and other instruments across the globe observed the other 30 FRBs, which illuminated the missing matter as they travelled great distances across the cosmos to reach Earth.
“The FRBs shine through the fog of the intergalactic medium, and by precisely measuring how the light slows down, we can weigh that fog, even when it’s too faint to see,” says lead author Liam Connor, assistant professor at Harvard.
The 69 FRBs observed in the study are located at distances ranging from 11.74 million to 9.1 billion light years from Earth. The object at the farthest end of that range, FRB 20230521B, is the most distant FRB on record. For the study, researchers needed to work with FRBs with known locations, which limited their pool as only roughly one hundred of the over one thousand FRBs have been traced to their source.
Viewing Matter Through a Cosmic Prism
On the long journey to Earth, light from FRBs spreads out into different wavelengths, in a manner similar to how a prism creates a rainbow. Where this becomes important to the new study is in how the amount of matter that the light comes into contact with affects the degree of light dispersion.
“It’s like we’re seeing the shadow of all the baryons, with FRBs as the backlight,” says co-author Vikram Ravi, an assistant professor of astronomy at Caltech. “If you see a person in front of you, you can find out a lot about them. But if you just see their shadow, you still know that they’re there and roughly how big they are.”
After analyzing the light dispersion, the researchers concluded that 76% of the matter in the universe resides in the intergalactic medium, the vast expanses between galaxies. The next most significant amount belongs to galaxy halos, accounting for approximately 15%. What remains makes up galaxies, in forms such as burning stars or cold gas.
Fast Radio Bursts Continue to Offer Clues
While the results of the new study do correlate with earlier theoretical expectations, this is the first time observations have confirmed the expected distribution of matter in the universe. Broadly, the new work will aid astronomers in understanding how FRBs can be used to address other cosmic questions, as well as providing new insights into galaxy growth.
Neutrinos, in particular, are an area where FRBs may be able to provide substantial clues. Those subatomic particles should have no mass according to the standard model of physics, yet a minuscule amount of mass has been detected in them. If FRBs can enable astronomers to measure the mass of neutrinos precisely, that data would be crucial to developing new physics that extends beyond the standard model.
Caltech is presently working on a follow-up to DSA-110, DSA-2000. From the Nevada desert, the radio telescope is expected to identify and locate 10,000 FRBs a year, filling in scientific understanding of the bursts and significantly enhancing their utility as matter probes.
The paper “A Gas-Rich Cosmic Web Revealed by the Partitioning of the Missing Baryons” appeared on June 16, 2025, in Nature.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.