Table of Contents
Scientists have long puzzled over why some cosmic structures form stars while others remain dark, with recent findings suggesting that “dark galaxies” containing gas and dark matter but few or no stars may be widespread throughout the universe. Finding these elusive objects could help researchers test leading models of galaxy formation and understand why certain haloes spark star creation while others do not.
The Hunt for Dark Galaxies
The prevailing cosmological model, known as Lambda-CDM, relies on two invisible components: dark energy, which drives the accelerating expansion of the universe, and cold dark matter, a hypothetical substance accounting for roughly 85 percent of cosmic matter. According to this framework, clumps of dark matter form haloes that act as gravitational scaffolding for galaxies. Gas gathers inside these haloes, eventually cooling and collapsing to create stars, though scientists are still working to understand why this star-forming process occurs in some haloes and bypasses others.
One theory is that certain haloes hold massive amounts of hydrogen gas yet generate very few stars, rendering them extremely difficult to detect. Now, researchers have identified a promising candidate for such a dark galaxy, publishing their findings in the Monthly Notices of the Royal Astronomical Society. Known as AGC 322753, the object holds approximately 2 billion times the mass of the sun in hydrogen and appears to lack stars entirely, although researchers note it is too large to be truly starless. If verified, it would represent an unusually massive example of a starless galaxy.
A Surprising Discovery and Potential Rotation
Researchers at Swinburne University of Technology in Australia uncovered the object by cross-referencing observations from two existing radio surveys: one conducted by the Arecibo Observatory in Puerto Rico and another by China’s Five-Hundred-Meter Aperture Spherical Radio Telescope. Both facilities detected a massive hydrogen cloud. While other researchers have identified potential dark galaxies over the past decade, this candidate stands out because it may be rotating.
Hydrogen gas emits radio waves at a specific wavelength that shortens when the gas moves toward Earth and stretches when it moves away. The signal from AGC 322753 displays two distinct peaks, indicating that one side of the gas cloud approaches Earth while the other recedes. This suggests the gas has settled into an organized, rotating structure resembling the discs of ordinary galaxies, despite seemingly forming no stars.
Marco Monaci, a study author at Swinburne University of Technology, stated that theoretical models and simulations indicate dark galaxies should not rotate, making the finding extremely interesting. If AGC 322753 is indeed a rotating disc devoid of stars, astronomers will need to explain how the gas organized into such a structure without generating them.
Weighing the Evidence and Alternative Explanations
Most galaxy formation models suggest a cloud containing this much hydrogen should produce at least a few stars as densest regions collapse under gravity and ignite. The study’s coauthors suggest that AGC 322753 very likely contains stars that are simply too faint to see, making it an “almost dark” galaxy. Deep optical images from the DESI Legacy Imaging Surveys, which search for starlight instead of radio waves from hydrogen, reveal no starlight at all. Based on this, the team calculates that AGC 322753 holds at least 40 times more mass in hydrogen than in stars, raising questions about what prevented star formation.
Additional observations using the Very Large Array, a network of radio antennas in New Mexico, will be required to confirm whether AGC 322753 is genuinely dark or nearly dark. Robert Minchin at the National Radio Astronomy Observatory noted that gathering more information is an essential next step because the discovery is intriguing.
Minchin is familiar with how misleading such signals can be, having participated in the discovery of VIRGOHI21, a dark galaxy candidate from the early 2000s that was later reinterpreted as a tidal tail—a long stream of hydrogen pulled from a nearby galaxy by gravity. While gas torn away in this manner can retain original motion and mimic rotation, Minchin points out there is no obvious nearby galaxy for AGC 322753. Rhys Taylor, an astronomer at the Astronomical Institute of the Czech Academy of Sciences in Prague, added that reproducing the candidate’s double-peaked profile through tidal debris is technically possible but fantastically unlikely.
However, Taylor and Minchin raised another concern: the two radio surveys placed the hydrogen at noticeably different positions in the sky. This discrepancy indicates that AGC 322753 might be an extended gas cloud rather than a compact galaxy, a question follow-up observations may resolve.
Elisabeth Adams at the Planetary Science Institute explained that whether AGC 322753 proves completely dark or exceptionally faint, studying such objects helps astronomers determine why certain galaxies form stars while others do not. Potential reasons include a galaxy spinning so rapidly that its gas remains too spread out to collapse, or ultraviolet light from early stars heating the gas and preventing condensation. Adams noted that once an object is no longer considered a dark galaxy, people tend to stop paying attention, an approach she believes is incorrect.
If AGC 322753 is confirmed as truly starless, it will rank among the most massive dark galaxies ever discovered, potentially explaining the whereabouts of missing galaxies predicted by Lambda-CDM. The finding could also clarify why certain gas-rich galaxies form stars while others remain dormant, offering a valuable test for galaxy formation models and fresh insights into the nature of dark matter.
