Scientists Use Cosmic Pulses to Map Universe's Missing Matter

Astronomers are utilizing mysterious, powerful flashes of energy from deep space to solve one of cosmology's most persistent puzzles: the location of a substantial portion of the universe's ordinary matter. These signals, known as Fast Radio Bursts (FRBs), are providing a new method to detect matter that has remained invisible to traditional telescopes.

This research is not only helping to balance the cosmic ledger but is also offering new insights into the evolution of galaxies, the formation of stars, and the behavior of supermassive black holes. By tracing these brief but brilliant pulses, scientists are beginning to construct a comprehensive map of the universe's large-scale structure.

A Cosmic Bookkeeping Problem

For decades, scientists have faced a significant discrepancy in their understanding of the universe's composition. Models of the Big Bang, particularly the widely accepted Lambda-Cold Dark Matter (Lambda-CDM) model, predict that baryonic matter should constitute about 5% of the total mass and energy in the cosmos. Baryonic matter is the "normal" stuff that makes up stars, planets, and people.

However, when astronomers tallied all the visible matter in galaxies and galaxy clusters, they could only account for a fraction of what was predicted. Approximately 30% of the expected baryonic matter was seemingly absent. This challenge became known as the "missing baryon problem."

"It's basically a cosmic bookkeeping problem," Manisha Caleb, an astrophysicist at the University of Sydney, explained to Live Science. The inability to find this matter was a persistent issue for cosmologists.

Liam Connor, an astrophysicist at Harvard University, highlighted the frustration within the scientific community. "It's kind of annoying and embarrassing to be missing much of the normal ordinary matter in the universe," he stated. Researchers theorized the missing material existed as a tenuous, hot gas spread thinly between galaxies, known as the warm-hot intergalactic medium (WHIM).

Detecting this medium proved extremely difficult because its diffuse nature means it emits very little light. Caleb compared the challenge to "trying to see fog in the dark." A new observational tool was needed to illuminate this invisible cosmic web.

Fast Radio Bursts Offer a Solution

The breakthrough came from an unexpected source: Fast Radio Bursts. First identified in 2007 by astronomer Duncan Lorimer, FRBs are incredibly powerful but brief flashes of radio waves originating from distant galaxies. A typical FRB lasts for only a few milliseconds yet can release more energy than the sun does over several days.

Initially, the focus of FRB research was on determining their origins. Most theories pointed to magnetars—highly magnetized, dense remnants of massive stars. However, a 2024 observation by McGill University doctoral student Vishwangi Shah and her team complicated this picture. They traced an FRB to the edge of a "dead" galaxy with no active star formation, suggesting that FRBs might be more common and produced by more diverse sources than previously thought.

This discovery reinforced an idea first proposed in 2013: FRBs could be used to find the missing baryons. As these radio waves travel across billions of light-years, they pass through the intergalactic medium. The free electrons in the baryonic gas slightly slow down the radio waves, with lower-frequency waves being delayed more than higher-frequency ones.

This method is uniquely suited for this task because FRBs are unaffected by dark matter. "As far as dark matter is concerned, the FRB doesn't exist, and vice versa," Connor explained. This allows for a clean measurement of only the ordinary matter scattered between galaxies.

Confirming Models and Locating Matter

In 2020, a team of scientists put this theory into practice. By analyzing the dispersion measures of a set of well-localized FRBs, they were able to estimate the universe's total baryonic density. Their calculations showed that baryons account for approximately 5% of the universe's total matter and energy, a figure that perfectly matched the predictions of the Lambda-CDM model.

This result was a powerful confirmation of both the model of the Big Bang and the utility of FRBs as cosmological probes. It demonstrated that the missing matter was indeed present and that the universe's composition has remained consistent since its early stages.

"If we can pin down where the missing matter is, we can build much better models of everything from how galaxies recycle gas to how elements get spread through the universe." - Manisha Caleb, University of Sydney

Subsequent research has provided even more detail. A study in June 2025 led by Connor used FRBs to estimate the location of this matter. Their findings suggested that about 76% of all baryons reside in the diffuse, ionized gas between galaxies, forming a vast "baryon cosmic web."

Building a 'Baryonic Google Maps'

Pinpointing the distribution of baryonic matter has profound implications for understanding the cosmos. It can reveal how supermassive black holes regulate their host galaxies, how stars form from interstellar gas, and how galaxies themselves grow and evolve over billions of years.

According to Julian Muñoz, a theoretical cosmologist at The University of Texas at Austin, this knowledge is crucial. If the predicted matter didn't exist, "it would mean something is wrong with our models," potentially indicating gaps in our understanding of cosmic history.

The primary limitation now is the number of FRBs whose origins have been precisely located. While thousands of bursts have been detected, only about 50 have been traced back to their host galaxy. "The next steps are about scale," Caleb said. "We need hundreds — ideally thousands — of well-localised FRBs so we can use them like pins in a cosmic map."

The Future of FRB Detection

Several large-scale projects are underway to dramatically increase the number of localized FRBs. These initiatives promise to transform the field from making individual discoveries to conducting large-scale statistical analysis of the universe's structure.

• CHIME (Canadian Hydrogen Intensity Mapping Experiment): This project is expanding by building three outrigger telescopes across North America. Working together, they will be able to pinpoint FRB locations with much greater precision as soon as they are detected.
• Deep Synoptic Array 2000 (DSA-2000): Located in Nevada, this next-generation radio telescope will feature nearly 20 times more antennae than its predecessor. It will survey the entire sky repeatedly, with the goal of finding and localizing over 10,000 FRBs per year.

With these powerful new instruments, astronomers are on the verge of creating a detailed, three-dimensional map of the universe's baryonic matter. Caleb envisions this as a "baryonic Google Maps," a tool that, when combined with maps of dark matter, will unveil the complete underlying architecture of the cosmos.

"This is just the beginning," she concluded, highlighting the immense potential of FRBs to continue revealing the universe's secrets.