Most Ordinary Matter Hides Between Galaxies — Fast Radio Bursts Complete the Census

Summary: New measurements using fast radio bursts (FRBs) show that most ordinary atoms in the universe are not locked inside stars or galaxies but distributed through the hot, diffuse gas between them. A June 2025 study of 69 FRBs observed with a 110‑dish radio array finds roughly 76% of ordinary matter in intergalactic space, 15% in galaxy halos and 9% inside galaxies.

Where Is the Universe’s Ordinary Matter?

If you point a telescope across the heavens you will see billions of galaxies, each with countless stars and often a massive central black hole. Those spectacular objects, however, do not contain most of the universe’s ordinary atoms — the protons, neutrons and electrons that make up hydrogen, helium and the elements of life.

The Big Bang model predicts that roughly 5% of the universe should be ordinary (baryonic) matter. For decades astronomers found far fewer atoms inside visible stars and galaxies than that prediction requires, producing a long‑standing deficit in the cosmic inventory.

The Cosmic Web and the Intergalactic Medium

The likeliest refuge for the "missing" atoms is the vast dark space between galaxies. Although commonly described as a vacuum, intergalactic space contains individual particles and atoms spread thinly across a filamentary structure called the cosmic web. The intergalactic medium (IGM) has an average density near one atom per cubic meter — about one atom every 35 cubic feet — and temperatures of millions of degrees. Even at this extreme rarity, over the observable universe’s enormous volume that material adds up.

Why the IGM Is Hard To Observe

Because the IGM is so hot it radiates primarily at short X‑ray wavelengths, and X‑ray telescopes are typically smaller and less sensitive than optical instruments. That makes a direct inventory of this gas difficult, which is why astronomers developed alternate techniques to trace the missing atoms.

Fast Radio Bursts: A New Probe

Fast radio bursts (FRBs) are millisecond‑duration flashes of radio waves first discovered in 2007. Many FRBs originate in distant galaxies, likely from highly magnetized neutron stars known as magnetars. As an FRB travels through space it interacts with free electrons in ionized gas: longer radio wavelengths are delayed more than shorter ones, spreading the pulse in frequency. By measuring that dispersion, astronomers can infer how much ionized gas the signal has passed through.

In a June 2025 study, researchers from Caltech and the Harvard‑Smithsonian Center for Astrophysics analyzed 69 FRBs recorded with an array of 110 radio dishes in California. Their dispersion measures allowed a direct accounting of ionized baryons along many sightlines.

Key Finding

The team’s results indicate that about 76% of ordinary matter resides in the intergalactic medium (the space between galaxies), roughly 15% is contained in galaxy halos (the diffuse gas surrounding galaxies), and only about 9% is bound up in stars and cold gas inside galaxies. This completes the census of baryonic matter predicted by Big Bang nucleosynthesis and strengthens confidence in the standard cosmological model.

What Comes Next?

Thousands of FRBs have now been detected, and upcoming radio arrays are expected to raise the detection rate to ~10,000 per year. Large samples will let FRBs map the three‑dimensional structure of the cosmic web, improving measurements of baryon distribution, large‑scale structure and intervening gas properties.

The Bigger Mystery Remains

Even after accounting for ordinary atoms, most of the universe’s energy budget remains mysterious: dark matter and dark energy dominate. Dark matter — inferred from gravitational lensing and galactic dynamics — likely outweighs ordinary matter by more than five to one, and dark energy drives the accelerating expansion of space. FRB science resolves a long-standing puzzle about baryons, but cosmology still faces deeper unknowns.

Article adapted from work by Chris Impey (University of Arizona) and originally published by The Conversation.

Funding disclosures noted in the original: NASA, NSF, Howard Hughes Medical Institute, Templeton Foundation.