Dark galaxy

A dark galaxy is a hypothesized galaxy with dark matter but no (or very few) 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 the existence of dark galaxies, but there are no confirmed examples to date. Dark galaxies are distinct from intergalactic gas clouds caused by galactic tidal interactions, which do not contain dark matter, so they do not technically qualify as galaxies. Distinguishing between intergalactic gas clouds and galaxies is difficult; most candidate dark galaxies turn out to be tidal gas clouds. The best candidate dark galaxies to date as of 2025 include HI1225+01, AGC229385, AC G185.0-11.5 (the most recent and promising candidate thus far), and numerous gas clouds detected in studies of quasars.

On 25 August 2016, astronomers reported that Dragonfly 44, an ultra diffuse galaxy (UDG) with the mass of the Milky Way galaxy, but with nearly no discernible stars or galactic structure, is made almost entirely of dark matter.

Large surveys with sensitive but low-resolution radio telescopes like Arecibo or the Parkes Telescope look for 21-cm emission from atomic hydrogen in galaxies. These surveys are then matched to optical surveys to identify any objects with no optical counterpart—that is, sources with no stars. Thus, atomic hydrogen observed by radio telescopes, within an identifiable compact object—without the presence of molecular gas or optical counterpart—suggests that the compact object is bound together by the gravitational field of an otherwise invisible dark matter cloud.

Another way astronomers search for dark galaxies is to look for hydrogen absorption lines in the spectra of background quasars. This technique has revealed many intergalactic clouds of hydrogen, but following up on candidate dark galaxies is difficult, since these sources tend to be too far away and are often optically drowned out by the bright light from the quasars.

In 2005, astronomers discovered gas cloud VIRGOHI21. They attempted to determine what it is, and why it exerts such a massive gravitational pull on galaxy NGC 4254. After years of ruling out other possible explanations, some have concluded that VIRGOHI21 is a dark galaxy.

The actual size of dark galaxies is unknown because they cannot be observed with normal telescopes. Rough estimations have ranged from double the size of the Milky Way to the size of a small quasar.

Dark galaxies are theoretically composed of dark matter, hydrogen, and dust. Some scientists support the idea that dark galaxies may contain stars. Yet the exact composition of dark galaxies remains unknown because there is no conclusive way to identify them. Nevertheless, astronomers estimate that the mass of the gas in these galaxies is approximately one billion times that of the Sun.

Dark galaxies contain no visible stars and are invisible to optical telescopes. The Arecibo Galaxy Environment Survey (AGES) harnessed the Arecibo radio telescope to search for dark galaxies, which are predicted to contain detectable amounts of neutral hydrogen. The Arecibo radio telescope was useful where others are not because of its ability to detect the emission from this neutral hydrogen, specifically the 21-cm line.