Invisible hydrogen clouds may hide the universe’s missing matter

Astronomers have chased a simple question for decades: where did a big chunk of normal matter go? A new map of galactic gas points to black holes pushing that missing matter out, very far from home.

A team combined two cosmic backlights to follow both invisible gas and the total mass that anchors it. In a new preprint, the researchers used data from millions of galaxies to compare where gas lingers and where dark matter dominates.

Where the missing matter hides

Normal matter, also called baryonic matter, makes up stars and planets, and includes the familiar protons, neutrons, and electrons. Only about 7 percent of this matter sits in stars, however, while the rest spreads out as dim gas.

Study lead author Boryana Hadzhiyska works at the University of California, Berkeley and the Lawrence Berkeley National Laboratory (LBL). Her group set out to track this diffuse gas without relying on its faint glow.

How the missing matter was found

The cosmic microwave background (CMB), the faint afterglow of the Big Bang that fills space with nearly uniform radiation, acts like a backlight for the universe. 

A technique called gravitational lensing makes use of the fact that light is bent by massive objects in the universe, like galaxies.

Gravitational lensing measures how the background radiation is warped by mass between us and the early universe, giving an independent mass map with a 43 sigma measurement.

Gas itself leaves a different sign. The kinematic Sunyaev-Zel’dovich effect measures a slight temperature shift in the cosmic background, caused by moving electrons in galactic gas.

This method analyzes the tiny temperature shifts in the background light, and their sensitivity to the total number of free electrons.

The galaxies came from DESI groups, for which the data was obtained using the Dark Energy Spectroscopic Instrument. This project catalogs both luminous red galaxies and a Bright Galaxy Sample drawn from wide field imaging.

Gas that surrounds galaxies

Near the center, the gas fraction falls well below the cosmic average, indicating powerful feedback that drives gas outward into the vastness of space.

The analysis reported mean halo masses of these galaxies. This is the total mass, including dark matter, that surrounds a galaxy.

The results showed a mean halo mass of around 10¹³ solar masses, suggesting that the gas extends farther from galaxies than the dark matter profile suggests. 

At larger radii, the gas fraction rises toward the expected universal value. That pattern implies that the gas was not lost, it was pushed into the outskirts where it becomes hard to see directly.

Evolution of galaxies and matter

The team also compared these measurements with state-of-the-art simulations that mimic how galaxies and matter evolve.

They found the simulations tend to predict too much gas inside the halos for these masses, a discrepancy larger than 4 sigma in several bins.

That mismatch tightens the case for stronger feedback than commonly assumed in current models. It also reduces a source of bias when cosmologists use small-scale structure to test fundamental physics.

How black holes push gas

If gas is more extended than expected, something has to move it. The evidence points to outflows powered by supermassive black holes at galactic centers, also known as active galactic nuclei (AGN). These are regions where a central black hole is feeding and releasing intense energy.

“We think that, once we go further away from the galaxy, we recover all of the missing gas,” said Hadzhiyska.

The black holes do not have to be blazingly bright to matter. Even intermittent activity can stir and expel gas, making it harder to cool and form new stars.

By lining up millions of systems, the team shows this is not a one off feature of a few dramatic galaxies. It is a population-level signal that is visible when individual systems are too faint to analyze alone.

Maps of gas and dark matter

Mass maps from ACT DR6 lensing provide a clean calibration of halo mass. That removes a major modeling uncertainty when interpreting gas measurements that depend on halo mass.

Other work that combines weak lensing with the same gas signal has pointed in the same direction, finding lower inner gas fractions than some X-ray based expectations. A recent analysis sharpened that point by jointly fitting lensing and kSZ data around galaxies.

Getting this right matters for precision tests of structure growth. If unmodeled feedback shifts gas around, it can mimic or hide other physical effects on small scales.

A tighter handle on gas also helps with the so-called clumpiness problem. This is the mismatch between how strongly matter clusters in the modern universe and what early universe data predict. Mapping where gas sits relative to dark matter is a direct way to reduce that confusion.

The search for missing matter

Future CMB surveys and deeper galaxy maps will push this method to smaller scales and higher redshifts. That will test how feedback evolves and whether the trends depend on galaxy type and environment.

Better velocity reconstructions and higher resolution maps will cut remaining uncertainties in the gas profiles. With that, models can be tuned to match the real universe rather than the other way around.

The technique is scalable. As data sets grow, the statistical power increases quickly, letting researchers split the sample by stellar mass, age, or environment without losing signal.

That is how a once-missing piece becomes a tool for new tests. The puzzle of the invisible half of ordinary matter is finally guiding us toward how galaxies grow and how black holes shape their neighborhoods.

The study is published in the preprint server arXiv.

Image Credit: NASA/CXC/M.Weiss; NASA/CXC/Ohio State/A Gupta et al

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