An artist’s depiction of the halo of hot hydrogen gas surrounding the Milky Way galaxy (center) and two satellite galaxies, the Large and Small Magellanic Clouds. The halo is more extended that astronomers originally thought, and contains enough hydrogen gas to resolve the problem of the universe’s missing baryonic mass. Credit: NASA/CXC/M.Weiss; NASA/CXC/Ohio State/A Gupta et al
For decades, scientists have known that ordinary matter — everything made of atoms — accounts for just 15% of the universe’s matter. The rest is mysterious dark matter. But even that modest slice didn’t fully add up. More than half of it was missing.
Now, by stacking millions of galaxies like poker chips, a team of astronomers has located what may be the universe’s long-lost matter—normal atoms, forged in the Big Bang, that had somehow gone missing from view.
The discovery, if confirmed, patches a major hole in our understanding of the cosmos.
According to physicists led by Boryana Hadzhiyska from the University of California, Berkeley, the matter is there in the form of very diffuse ionized hydrogen gas. We just couldn’t see it.
“We think that, once we go farther away from the galaxy, we recover all of the missing gas,” said Hadzhiyska. “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.”
A Backlight to the Universe
Ionized hydrogen is invisible to traditional telescopes. So, to spot this hidden matter, researchers turned to a technique that takes advantage of the oldest light in the universe: the cosmic microwave background, or CMB.
“The cosmic microwave background is in the back of everything we see in the universe. It’s the edge of the observable universe,” said Simone Ferraro, a senior scientist at Lawrence Berkeley National Laboratory and UC Berkeley.
By using this ancient light as a kind of cosmic backlight, the team measured how it subtly altered as it passed through clouds of ionized gas. This effect, known as the kinematic Sunyaev-Zel’dovich effect, occurs when CMB photons scatter off free electrons moving with galaxy clusters.
The researchers stacked images of around 7 million luminous red galaxies, observed with the Dark Energy Spectroscopic Instrument (DESI) in Arizona. Then they compared those images with ultra-precise measurements of the CMB taken by the Atacama Cosmology Telescope (ACT) in Chile.
Ultimately, the scientists found that the gas spread out more widely and faintly than previously believed, five times farther than astronomers once assumed.
Black Holes and Cosmic Feedback
A map of the cosmic microwave background radiation obtained by the Atacama Cosmology Telescope. The two circles highlight spots where ionized hydrogen gas has scattered the radiation. They leave a signature that can be used to estimate the amount of gas surrounding galaxies. Credit: ACT; Journal of Cosmology and Astroparticle Physics (2017).
This isn’t about solving some long-standing inventory problem. Finding this much hydrogen gas may have profound implications in astrophysics.
Astronomers have long thought that black holes become “active” — spewing out jets of matter and radiation — primarily during their formative years. These periods light up the centers of galaxies and create quasars. But if ionized gas is so widely distributed around mature galaxies, it may mean these black holes stir up gas more often than previously believed.
“One problem we don’t understand is about AGNs (active galactic nuclei or quasars), and one of the hypotheses is that they turn on and off occasionally in what is called a duty cycle,” said Hadzhiyska.
That activity creates what scientists call feedback: energy flowing from the galactic core into space, which in turn regulates the birth of stars. Earlier hints of such extensive feedback were reported in 2020, but the new study — with more galaxies and higher precision — strengthens the case.
The DESI collaboration made the largest 3D map of our universe to date and uses it to study dark energy. In this visualization, Earth is at the center, and every dot is a galaxy. Credit: DESI collaboration and KPNO/NOIRLab/NSF/AURA/R. Proctor
In fact, the new maps suggest that this gas doesn’t float randomly around galaxies but instead follows the cosmic web — a vast, filamentary network of matter that stretches across the universe.
“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.”
As the results undergo peer review at Physical Review Letters, astronomers are already looking at how to refine simulations to match this more vigorous picture of feedback and galactic life cycles. Some models have begun to rise to the challenge, baking in stronger outflows of gas.
For now, one of the most pressing questions in modern cosmology — where is the universe’s missing matter? — may finally have an answer. It was always there, quietly floating just beyond the edge of our sight.
