'Ghost gas' may solve the mystery of the universe’s missing matter

For decades, astronomers have searched for the universe’s missing matter – the normal matter that should exist according to the Big Bang theory, but can’t be fully seen.

This isn’t about dark matter, which constitutes most of the universe’s mass. It’s about ordinary, visible matter like stars, gas, and galaxies. Even after accounting for everything that can be observed through telescopes, more than half of this matter seemed to be missing.

But new research might have solved the mystery. Scientists believe they’ve found the missing matter in a form that’s been hiding in plain sight – very diffuse, invisible hydrogen gas floating far beyond what we once thought was the edge of galaxies.

Faint gas solves the mystery

The discovery comes from a team of researchers at the University of California, Berkeley, in collaboration with the Lawrence Berkeley National Laboratory.

The experts believe the missing matter is in a faint, expanded halo of ionized hydrogen that surrounds galaxies and stretches much farther than previously assumed – about five times farther, in fact.

This gas doesn’t glow or shine. It’s nearly impossible to detect with traditional tools. But its presence helps fix a major inconsistency between what we see in the sky and what our models of the Big Bang predict.

“We think that, once we go farther away from the galaxy, we recover all of the missing gas,” said Boryana Hadzhiyska, a Miller Postdoctoral Fellow at UC Berkeley and lead author of the study.

“To be more accurate, we have to do a careful analysis with simulations, which we haven’t done. We want to do a careful job.”

“The measurements are certainly consistent with finding all of the gas,” noted Simone Ferraro, a senior scientist at Berkeley Lab and UC Berkeley.

The cosmic microwave background

To find this gas, the team used a technique that involved “stacking” images of roughly 7 million galaxies. All of these galaxies lie within 8 billion light-years from Earth.

By studying how these galaxies slightly dim or brighten the cosmic microwave background – the oldest light in the universe – they were able to detect the scattering effect from free electrons in the ionized gas. This phenomenon is called the kinematic Sunyaev-Zel’dovich effect.

“The cosmic microwave background is in the back of everything we see in the universe. It’s the edge of the observable universe,” Ferraro explained. “So you can use that as a backlight to see where the gas is.”

The galaxy images used in the study were taken using the Dark Energy Spectroscopic Instrument (DESI) on the Mayall Telescope in Arizona. DESI, built by a Berkeley Lab-led collaboration, is working on a massive 3D map of the universe.

For background light, the team relied on data from the Atacama Cosmology Telescope in Chile, which produced the most accurate measurements of the cosmic microwave background before its retirement in 2022.

Gas expelled from black holes

The discovery has bigger implications than just solving the missing matter problem. It also sheds light on what’s happening at the center of galaxies – specifically, around the enormous black holes that lie at their cores.

Traditionally, scientists believed these black holes only shot out jets of gas and radiation early in their life, when they were feeding on surrounding material. But this new research suggests that the black holes might not be so quiet during their middle years after all.

“One problem we don’t understand is about AGNs, and one of the hypotheses is that they turn on and off occasionally in what is called a duty cycle,” said Hadzhiyska.

This process – called feedback – involves gas being expelled far from the galaxy and then falling back into the galactic disk. It plays a key role in regulating how new stars are formed. Earlier work in 2020 by Ferraro and his colleagues had already hinted at this extended feedback mechanism.

Graduate student Bernardita Ried Guachalla from Stanford University later confirmed the findings using a more nearby galaxy sample. Her work also revealed that the gas isn’t evenly spread around galaxies but rather follows the “cosmic filaments”- long strands of matter that connect galaxies across the universe.

Rethinking our universe models

The findings suggest that galaxy evolution models need a serious upgrade. Simulations of how galaxies form and change over time often don’t account for this much gas being pushed so far out by central black holes.

Some researchers are already adapting their models to better match this new evidence. The results also help clarify other long-standing puzzles in cosmology.

“Knowing where the gas is has become one of the most serious limiting factors in trying to get cosmology out of current and future surveys,” Ferraro said. “We’ve kind of hit this wall, and this is the right time to address these questions.”

“Once you know where the gas is, you can ask, ‘What’s the consequence for cosmological problems?'”

For instance, some assumptions suggest that gas follows dark matter. But if large amounts of gas are being blown out of galaxies, that idea needs to be re-evaluated. If not corrected, this can lead to inaccuracies in cosmological calculations. Now, this research might help resolve those issues.

“There are a huge number of people interested in using our measurements to do a very thorough analysis that includes this gas,” said Hadzhiyska. “People in astronomy care a lot about it for understanding galaxy formation and evolution.”

From missing matter to cosmic insights

Beyond helping explain current galaxies, this approach may open a new window into the early universe.

By using the same kinematic Sunyaev-Zel’dovich effect, scientists might be able to study large-scale structures that formed just after the Big Bang. It could even help test major theories about gravity and general relativity.

While there’s still work to be done, this research offers a promising step forward in answering some of cosmology’s biggest questions – starting with where half the universe’s matter has been hiding all along.

The full study was published in the journal arXiv.

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

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