Astronomers Use Space Flashes to Find Universe’s Missing Matter

Astronomers have solved one of the universe’s greatest mysteries by pinpointing the location of missing ordinary matter that has eluded scientists for decades.

Using fast radio bursts—brief, intense flashes of radio waves from distant galaxies—researchers at Harvard and Caltech mapped where three-quarters of the universe’s normal matter has been hiding. The answer surprised many: it’s floating in the vast spaces between galaxies, forming an invisible cosmic web that connects all the structures we see in space. This discovery represents the first time scientists have created a detailed census of where all the universe’s ordinary matter actually resides.

The study, published in Nature Astronomy, analyzed 60 fast radio bursts ranging from relatively nearby sources to the most distant ever recorded—nearly 9.1 billion light-years away. Each burst acted like a cosmic flashlight, illuminating the otherwise invisible gas that fills the universe.

Decades-Old Mystery Solved

“The decades-old ‘missing baryon problem’ was never about whether the matter existed,” said Liam Connor, the study’s lead author from the Center for Astrophysics | Harvard & Smithsonian. “It was always: Where is it? Now, thanks to FRBs, we know: three-quarters of it is floating between galaxies in the cosmic web.”

Scientists have long known that ordinary matter—composed primarily of protons and neutrons—makes up only about 5% of the universe, with dark matter and dark energy accounting for the rest. But even within that small fraction, at least half was mysteriously unaccounted for in traditional observations of stars, galaxies, and gas clouds.

Previous attempts to locate this missing matter relied on X-ray emissions and ultraviolet observations of distant quasars, which provided only hints of vast amounts of thin, warm gas scattered throughout space. The problem was that this intergalactic medium exists as hot, low-density gas that remains largely invisible to most telescopes.

Key Discoveries:

• 76% of ordinary matter resides in intergalactic space
• 15% exists in galaxy halos surrounding visible galaxies
• Only 9% remains in stars, planets, and cold galactic gas
• Fast radio bursts can precisely measure invisible matter
• Universe’s structure matches advanced computer simulations

Cosmic Flashlights Reveal Hidden Universe

Fast radio bursts work as cosmic measuring tools because different wavelengths of radio waves travel at slightly different speeds through ionized gas. By precisely measuring how much each burst slowed down during its journey across billions of light-years, scientists could calculate the total amount of matter the signal encountered.

“FRBs act as cosmic flashlights,” Connor explained. “They 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.”

The technique required unprecedented precision in both detecting the bursts and determining their exact origins. The Deep Synoptic Array-110 telescope in California proved crucial for this work, as it was specifically designed to locate fast radio bursts to their host galaxies with arcsecond accuracy.

Cosmic Thermostat Discovery

One significant finding absent from initial reports involves the discovery of what researchers call a “cosmic thermostat” effect. The study revealed that supermassive black holes and exploding stars efficiently blast gas out of galaxies and into the intergalactic medium, preventing galaxies from accumulating too much matter.

“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,” Connor noted. “Our results show this feedback must be efficient, blasting gas out of galaxies and into the IGM.”

This feedback mechanism explains why individual galaxies contain far less matter than theoretical models predicted. Instead of remaining gravitationally bound to galaxies, much of the ordinary matter gets expelled into the cosmic web through energetic processes like supernovae explosions and active galactic nuclei.

The Most Distant Signal Ever

The study’s crown jewel was FRB 20230521B, located 9.1 billion light-years away—the most distant fast radio burst ever confirmed. This ancient signal traveled for two-thirds the age of the universe before reaching Earth, carrying information about matter distribution from when the cosmos was much younger.

Remarkably, even this distant burst showed evidence for the cosmic matter distribution that researchers observed in nearby regions, suggesting the universe’s large-scale structure has remained relatively stable over billions of years.

What makes FRB 20230521B particularly valuable is its position near what scientists call the “DM cliff”—a sharp boundary below which no fast radio bursts are observed. This boundary exists because even the emptiest regions of space contain enough intergalactic matter to contribute a minimum signal delay.

Bridging Early and Late Universe

The research provides an independent measurement of the universe’s ordinary matter content that agrees remarkably well with predictions from the cosmic microwave background and Big Bang nucleosynthesis. Scientists measured the current baryon density at 10% precision, finding it matches early universe values within experimental uncertainty.

This agreement bridges a crucial gap between observations of the universe when it was only 380,000 years old and measurements of its current state 13.8 billion years later. The consistency supports our fundamental understanding of how matter evolved from the primordial universe to today’s cosmic web.

Implications for Cosmology

“It’s a triumph of modern astronomy,” said Vikram Ravi, assistant professor at Caltech and co-author of the study. “We’re beginning to see the Universe’s structure and composition in a whole new light, thanks to FRBs. These brief flashes allow us to trace the otherwise invisible matter that fills the vast spaces between galaxies.”

The findings may also help resolve the “S8 tension” in cosmology, where observations of cosmic structure appear inconsistent with theoretical predictions. If strong feedback processes efficiently move matter from galaxies into intergalactic space, it could explain why measured cosmic structures appear less dense than expected.

Revolutionary Mapping Technique

Beyond solving the missing matter mystery, the study establishes fast radio bursts as powerful tools for mapping cosmic structure. Unlike other techniques that rely on light-emitting objects, FRBs can probe the dark spaces between galaxies where most ordinary matter actually resides.

The cosmic web—the network of gas filaments connecting galaxies—has been largely theoretical until now. While computer simulations predicted its existence, direct observations remained elusive because the gas is too hot and diffuse to emit detectable light.

Fast radio bursts change this completely by providing a direct probe of matter along their entire journey from source to Earth. Each burst essentially weighs every bit of ionized gas between its origin galaxy and our telescopes.

Golden Age of Discovery

“We’re entering a golden age,” Ravi declared. “Next-generation radio telescopes like the DSA-2000 and the Canadian Hydrogen Observatory and Radio-transient Detector will detect thousands of FRBs, allowing us to map the cosmic web in incredible detail.”

Future surveys could detect tens of thousands of fast radio bursts, enabling three-dimensional mapping of the cosmic web’s structure and evolution over cosmic time. This would provide unprecedented insights into how galaxies form, how matter flows through the universe, and how cosmic feedback processes shape the distribution we observe today.

The technique could also constrain fundamental properties like the Hubble constant and dark energy by precisely measuring how fast the universe expands at different epochs. With enough fast radio bursts, astronomers might create the most detailed map of cosmic matter ever attempted.

Rewriting Textbooks

For decades, astronomy textbooks have included estimates and educated guesses about where the universe’s ordinary matter might be hiding. This study provides the first definitive answer: it’s not missing at all, but rather distributed exactly where advanced computer simulations predicted it should be.

The cosmic web contains roughly four times more ordinary matter than all the visible galaxies, stars, and gas clouds combined. This invisible scaffolding provides the framework upon which all cosmic structures form and evolve.

As more powerful telescopes come online, fast radio bursts may become routine tools for cosmic archaeology, allowing scientists to trace the universe’s matter distribution back to its earliest epochs and forward to its ultimate fate.