Astronomers have identified one of the most compelling dark galaxy candidates to date: an object called AGC 322753 that holds roughly 2 billion times the Sun’s mass in hydrogen yet shows no detectable starlight. What makes the find especially provocative is that the gas may be rotating in an organized disc — a behavior that theoretical models say starless dark galaxies should not exhibit. The research, led by Swinburne University of Technology in Australia, has been published in the Monthly Notices of the Royal Astronomical Society.
- Massive gas reservoir: AGC 322753 contains neutral hydrogen equivalent to about 2 billion solar masses, located roughly 310 million light-years from Earth.
- Possible rotation: The hydrogen signal shows two distinct peaks, hinting that one side of the cloud approaches us while the other recedes, like a spinning disc.
- Not fully dark: The object is too massive to be completely starless, so the team suggests it may be an “almost dark” galaxy with stars too faint to detect.
- Confirmation pending: Follow-up observations with the Very Large Array in New Mexico are needed to rule out a tidal tail or an extended gas cloud.
Why a starless galaxy matters
Under the standard Lambda-CDM cosmological model, galaxies assemble inside clumps of cold dark matter known as haloes. Dark matter is thought to make up about 85 percent of the universe’s matter, providing the gravitational scaffolding within which gas cools and collapses to ignite stars. Yet astronomers still cannot fully explain why this process occurs in some haloes but not others. One possibility is that certain haloes contain vast quantities of hydrogen while forming very few stars, rendering them nearly invisible. Finding such objects would test the leading models of galaxy formation and help explain why some galaxies light up while others remain dark.
The discovery came from comparing two existing radio surveys: the ALFALFA survey conducted at the Arecibo Observatory in Puerto Rico, and the FASHI survey from China’s Five-Hundred-Meter Aperture Spherical Radio Telescope (FAST). Both had picked up the same enormous hydrogen cloud. While several dark galaxy candidates have emerged over the past decade, this one stands apart because of its possible rotation.
A rotating disc without stars?
Hydrogen gas emits radio waves at a signature wavelength that shortens when the gas moves toward Earth and stretches when it moves away. The signal from AGC 322753 displays two distinct peaks, suggesting that one side of the cloud is approaching while the other recedes — a pattern consistent with an organized, rotating structure much like the discs of ordinary galaxies. According to the supporting data, the line width is approximately 85 kilometers per second.
“The theoretical models and the simulations show us that dark galaxies shouldn’t rotate, so this is extremely interesting,” says Marco Monaci, an author of the study at Swinburne University of Technology.
If AGC 322753 really is a rotating disc with no stars, astronomers will have to explain how the gas organized itself into such a structure without producing any. That is not the only puzzle. Most galaxy formation models predict that a cloud holding this much hydrogen should have formed at least a few stars as its densest patches collapsed under their own gravity and ignited. The coauthors therefore suggest it is unlikely that the object contains absolutely no stars. Instead, they believe it may be an “almost dark” galaxy whose stars are too faint to see.
If stars are present, they must be extremely dim: even deep optical images from the DESI Legacy Imaging Surveys, which search for starlight rather than hydrogen, reveal none. From this, the team estimates that AGC 322753 contains at least 40 times more mass in hydrogen than in stars, raising the question of what suppressed star formation.
Caution and the VIRGOHI21 precedent
The researchers are careful not to overstate the finding. The two radio surveys placed the hydrogen at slightly different positions in the sky, leaving open the possibility that the object is an extended gas cloud rather than a compact galaxy. Another concern is that the structure could be a tidal tail — a long stream of hydrogen torn from a nearby galaxy by gravity. Gas stripped in this way can retain some of its original motion and mimic the signature of rotation.
Robert Minchin of the National Radio Astronomy Observatory knows how misleading such signals can be. He was involved in the discovery of VIRGOHI21, a dark galaxy candidate found in the early 2000s that was later reinterpreted as a tidal tail pulled from a neighboring galaxy. Minchin says more observations are needed to rule out the same explanation for AGC 322753, though there is no obvious nearby galaxy it could have come from.
“Finding out more about it really is an essential next step at the moment. It’s intriguing, and I want to know more,” says Minchin.
Astronomer Rhys Taylor of the Astronomical Institute of the Czech Academy of Sciences in Prague adds that tidal debris is unlikely to explain the observations, noting that reproducing the candidate’s distinctive double-peaked profile through such a process would be difficult.
Further observations, particularly with the Very Large Array in New Mexico, will be needed to confirm whether AGC 322753 is truly a dark galaxy or an almost dark one. Other possibilities under consideration include gas rotating very rapidly, or ultraviolet radiation from the first stars in the early universe overheating the gas and preventing it from condensing.
The discovery is a striking reminder of how science advances: a candidate this massive, this gas-rich and potentially rotating would directly challenge assumptions about how galaxies are born. Yet the VIRGOHI21 precedent weighs heavily, and the fact that one of the astronomers involved in that earlier episode is now urging caution is telling. The honest position is to celebrate the candidate without treating it as confirmed. Its possible rotation makes it especially intriguing, and the next round of observations will determine whether this is a genuine dark galaxy or another deceptive cloud of gas. The excitement is justified; certainty is not yet.