Astronomers have discovered that previously unaccounted-for hydrogen gas, comprising half of the universe's normal matter, exists as an extensive halo around galaxies. This invisible ionized gas was measured through a study spearheaded by Boryana Hadzhiyska at the University of California, Berkeley, and Simone Ferraro at Lawrence Berkeley National Laboratory. The findings were presented at scientific conferences and are under review by Physics Research Letters.

The research suggests these halos are more expansive than initially thought, challenging assumptions about the activity of massive black holes and their influence on galactic structure. Hadzhiyska noted, "We think that, once we go further away from the galaxy, we recover all of the missing gas." Simone Ferraro added, "The measurements are certainly consistent with finding all of the gas."

This new understanding addresses discrepancies between observed astronomical data and the established models of the universe's evolution. The discovery implies that black holes may expel gas beyond the galactic center, contradicting the assumption that this occurs primarily in their formative years.

The galaxy images used in the study were collected by the Dark Energy Spectroscopic Instrument (DESI) and Kitt Peak's Mayall 4-meter Telescope, while cosmic microwave background data was sourced from the now-decommissioned Atacama Cosmology Telescope (ACT) in Chile. Ferraro described the cosmic microwave background as "the edge of the observable universe," which serves as a backlight to reveal where the gas resides.

The study's method involved stacking images of about 7 million galaxies to measure the dimming or brightening of the cosmic microwave background, attributed to electron scattering within the ionized gas. This approach uncovered the extended distribution of hydrogen, necessitating updates to galaxy evolution models to include this intense feedback.

Hadzhiyska commented, "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." The revelations challenge the view that gas strictly follows dark matter patterns, an assumption that could introduce errors in cosmological models if uncorrected.

The DESI collaboration, which generated the largest 3D map of the universe to date, is funded by the U.S. Department of Energy, while Kitt Peak is managed by the National Optical-Infrared Astronomy Research Laboratory and supported by the National Science Foundation. The ACT was funded by the NSF and operated by Princeton University.