Cloud-9 is a mysterious structure in the depths of the Universe. Scientists suspect that it is a so-called “dark galaxy” — a cloud of gas with a mass millions of times greater than that of the Sun, compressed by the gravitational influence of dark matter. Recent studies have shown that it indeed contains no stars.
Galaxies Dominated by Dark Matter
Astronomers have carried out one of the deepest observations to date of Cloud-9, an object considered to be a “dark galaxy,” and found no traces of stars. As reported by phys.org, the paper presenting the results of this discovery was published on the arXiv preprint server on August 21.
The generally accepted model of the Universe — the Lambda-Cold Dark Matter model — predicts that the Universe should contain a large number of small, low-mass dark matter clumps that contain gas but have never managed to form stars. This is thought to be because the balance between gravity, which tries to compress the gas, gas cooling, and heating from the ultraviolet background — diffuse radiation from quasars and stars throughout the Universe — prevents the gas in these small halos from condensing into stars. These galaxies that cannot form stars are “dark” or “starless” galaxies, and the detection of one would confirm a key prediction of standard cosmology.
Cloud-9 was a promising candidate. It is a gas cloud near the spiral galaxy M94, located at a distance of about 4.66 megaparsecs, or roughly 15 million light-years. It is estimated to contain about 1 million solar masses of hydrogen, but no obvious stars had been detected in previous studies. However, earlier attempts to completely rule out the presence of stars in Cloud-9 were unsuccessful.
“Alternative interpretations of starless hydrogen clouds have been proposed, including tidal debris, high-velocity clouds, and transient gaseous structures,” the team writes in the paper. Some of these alternative scenarios may contain very faint stellar components, so deep imaging is needed to determine whether they are truly devoid of stars.
Imaging Cloud-9 Through Five Filters
In this study, a team of scientists led by Ignacio Trujillo of the Instituto de Astrofísica de Canarias used the high-speed HiPERCAM camera installed on the Gran Telescopio Canarias (GTC), one of the world’s largest optical telescopes, to image Cloud-9 simultaneously through five filters. The observations lasted for two nights in June 2026.
To maximize sensitivity to any faint, diffuse stellar signal, the researchers systematically changed the camera orientation between exposures and masked the region centered on Cloud-9 before subtracting the background. This ensured that any genuine stellar emission was not accidentally removed during data processing. The observations reached a limiting surface brightness of 31.4 magnitudes per square arcsecond in the g band — deep enough to detect extremely faint stellar populations invisible in previous surveys.
A Candidate Starless Galaxy
Thanks to these ultra-deep observations, the team detected no stellar light in the central region of the object. This non-detection set an upper limit of only 16,000 solar masses of stars within a region about 4,200 light-years in diameter. For comparison, Cloud-9 contains about 1 million solar masses of hydrogen, meaning there is at least 60 times more gas than stars, if any stars exist at all. This is an exceptionally small stellar content for an object that appears to have the properties of a galaxy.
The team concludes that this result “supports the hypothesis that Cloud-9 is an excellent candidate for a starless galaxy.”
This result lies between two previous estimates: an earlier upper limit based on a survey that “was not designed to preserve low-surface-brightness signals,” and a more detailed star-count estimate using the Hubble Space Telescope (HST), which relied heavily on a comparison galaxy that may be too compact. The new observations independently support earlier evidence that Cloud-9 may be a rare starless galaxy — a system dominated by gas and dark matter that, for some reason, failed to form stars.
