Astronomers have found a vast reservoir of hydrogen with something conspicuously missing: detectable starlight. The object, called AGC 322753, contains roughly 2 billion times the sun’s mass in neutral hydrogen, a potential supply of material for making stars. Its radio signal suggests the gas may be rotating in a disk, yet deep astronomical images reveal no matching glow.
Marco Monaci and colleagues at Swinburne University of Technology in Australia describe the object in Monthly Notices of the Royal Astronomical Society. Located about 310 million light-years away, it is a promising “dark galaxy” candidate—a system with very few stars, or perhaps none. Its enormous gas supply makes that apparent darkness especially difficult to explain.
Two detections, one missing galaxy
The team compared dark-galaxy candidates from the ALFALFA survey, conducted with the now decommissioned Arecibo radio telescope, against detections from a survey using China’s FAST telescope. Both surveys pick up radio emission from neutral hydrogen, allowing astronomers to find gas even where stars are too faint to see.
Between the catalogs, the researchers found 13 sources that matched. After checking nearby galaxies, image quality and possible visible counterparts, they singled out AGC 322753.
Its two radio detections agreed closely in velocity and signal shape. Separate telescopes observing it at different times provided strong evidence that the signal came from space rather than radio interference. Both observations also contained two peaks, a pattern consistent with gas rotating in a disk, and although the observations cannot yet map that disk, they do suggest an organized system.
The overlapping areas observed by the telescopes narrowed down the most likely location, giving the team a smaller patch of sky in which to search for stars.
How many stars could be hiding?
A blank patch in an image doesn’t automatically mean an absence of stars, so to test what the images might have missed, the researchers added artificial galaxies with different sizes and stellar masses, then checked which areas remained visible.
Those experiments suggested a rough upper limit of 50 million solar masses in stars. Under those assumptions, AGC 322753 contains at least 40 times more neutral hydrogen than stars. The limit depends on how widely the stars are spread and whether foreground glare masks their light. That leaves room for the presence of an exceptionally faint galaxy.
The team found that AGC 322753 contains more hydrogen than current models predict for a completely starless system. It “could have a stellar mass too low to be detectable in the Legacy Surveys,” the authors write.
There is a precedent for finding the missing light. Another proposed dark galaxy was reclassified after deep follow-up observations revealed a faint population of stars associated with its gas.
AGC 322753 also has an alternative explanation: It could be gas pulled from neighboring galaxies by gravitational interactions. The nearest known luminous neighbor is about 950,000 light-years away in projected separation. That distance, the large gas mass and the apparent rotation argue against such an origin, although they cannot rule it out.
Sharper radio observations could establish whether the gas really forms a rotating disk, while deeper optical images could reveal hidden stars. Finding them would still leave an intriguing galaxy to explain: one holding an enormous supply of gas, with remarkably little starlight to show for it.
https://phys.org/news/2026-10-fast-arecibo-dark-galaxy-vast.html
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