For decades, astronomers have known something didn’t add up in our universe.
The amount of “normal” matter—stars, gas, and galaxies—seen in the universe today doesn’t match what should exist based on calculations from the Big Bang.
In fact, more than half of this normal matter seemed to be missing.
Now, scientists have found that missing half in the form of invisible hydrogen gas floating around galaxies in vast, puffy halos.
This discovery helps solve a long-standing cosmic mystery and also sheds new light on how galaxies evolve.
The research, led by Boryana Hadzhiyska at the University of California, Berkeley, was recently presented at scientific meetings and is undergoing peer review.
Her work, alongside collaborator Simone Ferraro of Lawrence Berkeley National Laboratory, suggests that the hidden matter is in the form of ionized hydrogen gas that is much more spread out around galaxies than previously thought.
This gas is invisible to ordinary telescopes, but the team used a clever technique to detect its presence.
They measured how this ionized gas scattered light from the cosmic microwave background (CMB)—the faint afterglow of the Big Bang that fills the entire universe.
When this radiation passes through gas, it can be slightly dimmed or brightened in a way that gives clues about what the gas is doing. This effect is known as the kinematic Sunyaev-Zel’dovich effect.
To see this subtle signal, the team combined—or “stacked”—images of nearly 7 million galaxies to make the faint effect visible. These galaxies, known as luminous red galaxies, were observed by the Dark Energy Spectroscopic Instrument (DESI) in Arizona, while measurements of the CMB came from the Atacama Cosmology Telescope in Chile.
The results showed large halos of ionized hydrogen gas surrounding the galaxies, extending far beyond what scientists had expected.
This not only helps solve the mystery of the missing hydrogen but also suggests that black holes at the centers of galaxies may be more active over time than previously believed.
Until now, astronomers thought that black holes only expelled gas during a short period in a galaxy’s early life. But this new evidence suggests that powerful bursts of gas may happen repeatedly, affecting how stars form and how galaxies evolve.
The findings also show that this gas doesn’t sit evenly around galaxies. Instead, it follows a vast web of cosmic filaments that stretch across the universe, connecting galaxies in a kind of three-dimensional network.
This discovery could lead to updates in our models of how galaxies form and grow. It also offers a new tool to study the early universe. Scientists hope the same technique can be used to look deeper into cosmic history, revealing more about the universe’s structure, gravity, and how it evolved over billions of years.
Understanding where the gas is has become one of the most important questions in cosmology. These findings not only close a major gap in our knowledge but may also help resolve other puzzles, such as why the universe isn’t as “clumpy” as some models predict.
For now, the missing matter is missing no more—it’s just been hiding in plain sight, spread far and wide in the halos of galaxies across the cosmos.
