Astronomers Find Most Ordinary Matter as Diffuse Gas Between Galaxies

Astronomers analyzing a sample of 69 precisely located fast radio bursts published a census on 16 June 2025 estimating that 76 percent of ordinary matter resides as diffuse gas between galaxies. The findings help researchers isolate cosmological signals of dark matter, dark energy, and neutrino masses.

Cosmologists have long faced an inventory problem when trying to account for the universe’s baseline building blocks. While early universe measurements and Big Bang nucleosynthesis tightly constrain how many baryons should exist, visible reservoirs like shining stars, cold galactic gas, and hot cluster gas historically fell short of that prediction. Now, researchers turning brief flashes of radio light into cosmic backlights have mapped out where that missing material hides, opening a new window onto the invisible architecture of the cosmos.

Weighing the Cosmos Through Plasma Delays

Fast radio bursts, or FRBs, are intense millisecond-duration flashes of radio waves that originate from distant galaxies and travel billions of light-years to reach Earth. Their exact physical triggers remain shrouded in mystery, though the brief blasts are currently thought to erupt from rapidly rotating dead stars with immense magnetic fields known as magnetars. As these signals cross space, they encounter diffuse clouds of gas and dust. Free electrons within that ionized plasma slow down lower radio frequencies more than higher ones, creating a frequency-dependent timing lag known as a dispersion measure.

That timing signature acts as an unblinking cosmic measuring stick. Using that, we can quantify how much gas it interacted with along the line of sight.

Mapping the Baryon Budget Across 69 Localized Bursts

To turn those dispersion measures into an actual inventory, astronomers require securely identified host galaxies so they can calculate distances and redshifts.

The resulting census estimates that roughly 76 percent of ordinary matter sits outside virialized galactic halos as extremely diffuse ionized gas within the intergalactic medium, carrying an uncertainty of plus 10 and minus 11 percentage points. About 15 percent occupies extended galactic halos, leaving just 9 percent inside galaxies as stars and cold gas.

Untangling Galactic Feedback From Dark Matter and Neutrinos

Beyond counting missing atoms, the radio bursts provide a direct method to measure how matter clumps together on a grand scale. Scientists study that clumping to understand dark energy, the repulsive force driving the accelerated expansion of the universe, and dark matter, the invisible substance that outweighs ordinary matter. They also hope to pin down the mass of neutrinos, the elusive subatomic particles nicknamed ghost particles.

Astronomers Find Most Ordinary Matter as Diffuse Gas Between Galaxies

Yet a confounding factor complicates those cosmological measurements. Supermassive black holes at the centers of galaxies and exploding supernovae pump immense energy into their surroundings, pushing hot gas outward and smoothing out the matter surrounding galaxies in a process known as galactic feedback. It smooths out clumps of matter in a way that looks astonishingly similar to what massive neutrinos do, or what dark energy or dark matter theories predict. Unless scientists can independently measure this contribution from feedback, they can’t tell these effects apart.

A Weaker Suppressing Effect and Future Horizons

By analyzing a sample of about 100 FRBs, the Caltech-led team directly measured the impact of galactic feedback on cosmic clumpiness for the first time. The analysis shows that while feedback does smooth surrounding material, making it less clumpy, that suppressing effect is weaker than earlier estimates derived from X-ray and microwave telescopes like the eROSITA telescope and the Atacama Cosmology Telescope.

76% of Ordinary Matter Hides Between Galaxies. What Is It and What Else Is Out There

Researchers expect the field to expand dramatically in the coming years. With Caltech’s Deep Synoptic Array scheduled to begin operations in Nevada in 2029, observatories are projected to detect tens of thousands of fast radio bursts. When paired with precision sky surveys from instruments like the Euclid space telescope and the Vera Rubin Observatory, these transient radio flashes promise a sharper view of the invisible universe.

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