Astronomers Finally Track Down the Universe’s Missing Matter

For decades, astronomers have puzzled over a frustrating cosmic mystery: where exactly is all the ordinary matter in the universe? While dark matter — the invisible stuff detectable only by its gravitational fingerprints — has long dominated headlines, a more straightforward issue remained unresolved. We’ve known what ordinary matter is, the atoms and particles making up everything from stars to people to chocolate chip cookies, but for years, about half of it simply seemed to be missing.

Now, thanks to the help of some of the universe’s most powerful and fleeting phenomena — fast radio bursts (FRBs) — astronomers believe they’ve finally located this missing matter. And as it turns out, it was lurking in the immense, sparsely populated stretches between galaxies, in the form of wispy, diffuse gas.

A team of researchers led by Harvard astronomer Liam Connor detailed their findings this week in Nature Astronomy. By analyzing how radio waves from distant FRBs travel across the universe, the team was able to measure the amount of matter these signals encountered along their path to Earth. This breakthrough has allowed them not just to detect the elusive baryonic matter, but also to map where it resides in the vast cosmic wilderness.

A Foggy Web Between Galaxies

The majority of this ordinary matter — about 76%, according to the study — exists in the intergalactic medium, a thin fog of plasma that stretches across the spaces between galaxies. Another 15% is held within the extended halos of gas surrounding galaxies like our own Milky Way. The remaining 9% is concentrated inside galaxies themselves, mostly as stars, gas, and dust.

“We knew the total amount of ordinary matter from calculations based on ancient light left over from the Big Bang,” said Connor. “But for years, the challenge was actually finding where half of it was hiding. Now we know — it’s spread out in a wispy cosmic web, far from galaxies.”

This matter isn’t static. Violent events like supernova explosions and the activity of supermassive black holes can eject vast amounts of gas into intergalactic space. Instead of every proton and neutron ending up in a star, as one might imagine in a simpler universe, these powerful processes have scattered ordinary matter across unimaginable distances.

Detecting the Undetectable

So how do you find something so thinly spread it might as well be invisible? That’s where FRBs come in. These intense, millisecond-long blasts of radio waves come from distant points in the universe, and although their origins remain somewhat mysterious, they’re believed to be produced by highly magnetized neutron stars called magnetars — remnants of massive stars gone supernova.

As these radio signals travel toward Earth, they pass through clouds of plasma, where electrons cause the waves to disperse, spreading them out into different frequencies. The greater the amount of matter in the signal’s path, the more dispersed the radio waves become. By carefully measuring this effect across dozens of FRBs, scientists can estimate the amount of intervening matter, even in the most desolate regions of space.

For this study, researchers used signals from 69 FRBs. Thirty-nine of these were detected using Caltech’s Deep Synoptic Array, a network of 110 radio telescopes at Owens Valley Radio Observatory in California. The other 30 were picked up by other radio observatories around the globe. Some of the signals originated as far as 9.1 billion light-years away — the most distant FRBs measured to date.

Accounting for Every Atom

With this method, astronomers could finally confirm that the missing half of the universe’s ordinary matter was indeed out there, scattered in the intergalactic void. Having completed this census, scientists are eager to turn their attention to even bigger puzzles.

“We can now move on to even more important mysteries regarding the ordinary matter in the universe,” Connor noted. “And beyond that — what is the nature of dark matter, and why is it so difficult to detect directly?”

A Universe of Open Questions

While this discovery represents a significant milestone in cosmology, it’s far from the final chapter. Scientists still don’t fully understand how FRBs are generated, though magnetars remain the leading theory. And, of course, dark matter — the invisible mass making up roughly 85% of the universe’s total matter content — continues to evade direct detection, even as its gravitational influence shapes galaxies and clusters on the largest scales.

Interestingly, earlier this year, a team from the Canadian Hydrogen Intensity Mapping Experiment (CHIME) reported the detection of an FRB originating from a neighboring galaxy, further fueling interest in these enigmatic signals. As radio astronomy technology improves, especially with upcoming projects like the Square Kilometre Array (SKA) — poised to be the world’s largest radio telescope — scientists anticipate not only uncovering the full story of ordinary matter but also peeling back layers of the universe’s deepest secrets.

For now, it’s one more mystery solved in a universe that seems to have an infinite supply of them.