Dark galaxy
Hypothetical galaxies of dark matter, invisible to optical telescopes.
Wikipedia / Wikimedia Commons
A dark galaxy is a hypothesized galaxy composed primarily of dark matter, with no or very few stars. They are invisible to optical telescopes but may be detectable through their neutral hydrogen gas via radio observations. As of 2025, no confirmed examples exist, though several candidates have been proposed.
- field
- Astronomy
- known_for
- Hypothesized galaxies with dark matter but no visible stars
- composition
- Dark matter, hydrogen, dust (theoretically)
- detection_method
- 21-cm radio emission, hydrogen absorption lines in quasar spectra
- best_candidates
- HI1225+01, AGC229385, AC G185.0-11.5, VIRGOHI21, Smith's Cloud
Lore & Background
Dark galaxies were theorized as a class of object that could contain significant amounts of dark matter and gas but lack stars. They received their name because they have no visible stars but may be detectable if they contain significant amounts of gas. Astronomers have long theorized their existence, but no confirmed examples exist to date. Distinguishing dark galaxies from intergalactic gas clouds caused by galactic tidal interactions is difficult; most candidate dark galaxies turn out to be tidal gas clouds.
In 2005, astronomers discovered the gas cloud VIRGOHI21 and attempted to determine why it exerts such a massive gravitational pull on the galaxy NGC 4254. After years of ruling out other explanations, some concluded it is a dark galaxy. However, further observations revealed that VIRGOHI21 was an intergalactic gas cloud stripped from NGC 4254 by a high-speed collision caused by infall into the Virgo Cluster. Other candidates include Smith's Cloud, HVC 127-41-330, and J0613+52, discovered with the Green Bank Telescope when it was accidentally pointed to the wrong coordinates.
Reader's Guide
Dark galaxies are significant because they represent a predicted but unconfirmed class of object that could help explain galaxy formation and the distribution of dark matter. Scientists say that the galaxies we see today only began to create stars after dark galaxies, and that dark galaxies played a big role in many of the galaxies astronomers see today. Martin Haehnel claims that the precursor to the Milky Way was actually a much smaller bright galaxy that had merged with dark galaxies nearby. Sebastian Cantalupo agrees, stating that dark galaxies bring to big galaxies a lot of gas, which then accelerates star formation. The search for dark galaxies also teaches about the cosmic web—invisible filaments of gas and dark matter believed to permeate the universe. On 25 August 2016, astronomers reported that Dragonfly 44, an ultra diffuse galaxy with the mass of the Milky Way but nearly no discernible stars, is made almost entirely of dark matter. In June 2025, astronomers reported CDG-2, another ultra diffuse galaxy with only four galactic clusters, also made almost entirely of dark matter.
Did You Know?
- Dark galaxies are distinct from intergalactic gas clouds caused by galactic tidal interactions, which do not contain dark matter.
- The best candidate dark galaxies as of 2025 include HI1225+01, AGC229385, and AC G185.0-11.5.
- Dragonfly 44, an ultra diffuse galaxy with the mass of the Milky Way, is made almost entirely of dark matter.
- VIRGOHI21 was initially thought to be a dark galaxy but was later found to be an intergalactic gas cloud stripped from NGC 4254.
Frequently Asked Questions
What is a dark galaxy?
A dark galaxy is a theoretical galaxy made up almost entirely of dark matter, containing little to no starlight of its own. Because it lacks visible stars, it cannot be seen with optical telescopes, making it effectively invisible to the naked eye and standard imaging.
How do astronomers try to detect dark galaxies?
Since these objects emit no visible light, researchers look for their neutral hydrogen gas using 21-centimeter radio observations. They also search for hydrogen absorption lines in the spectra of distant quasars, which can reveal the presence of a gas-rich halo where no stars are shining.
Has any dark galaxy been confirmed as of 2025?
No confirmed example exists yet. Several candidates—such as HI1225+01, AGC229385, AC G185.0-11.5, VIRGOHI21, and Smith's Cloud—have been proposed, but each still awaits definitive proof that it lacks a stellar population.
What would a dark galaxy be made of?
Theoretically, its mass would be dominated by dark matter, with trace amounts of neutral hydrogen gas and possibly dust. Without stars to generate heat or light, the object would remain cold and dark across the electromagnetic spectrum.
Why are dark galaxies important to the field of astronomy?
If confirmed, they would provide direct evidence that dark matter can form galaxy-scale structures on its own, reshaping our understanding of how galaxies assemble. They also offer a natural testbed for dark-matter models that current visible-galaxy observations cannot fully constrain.
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