A dark galaxy is a hypothesized galaxy with 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 dark galaxies to date. Dark galaxies are distinct from intergalactic gas clouds caused by galactic tidal interactions, since these gas clouds do not contain dark matter, so they do not technically qualify as galaxies.
In 2000, astronomers found gas cloud VIRGOHI21 and attempted to determine what it was and why it caused such a gravitational pull from galaxy NGC 4254. After years of running out of other explanations, some have concluded that VIRGOHI21 is a dark galaxy, due to the massive effect it had on NGC 4254.
The actual size of dark galaxies is unknown because they cannot be observed with normal telescopes. There have been various estimations, ranging from double the size of the Milky Way to the size of a small quasar.
Dark galaxies are composed of dark matter. Furthermore, dark galaxies are theoretically composed of hydrogen and dust. Some scientists support the idea that dark galaxies may contain stars. Yet the exact composition of dark galaxies is unknown because there is no conclusive way to spot them so far. However, astronomers estimate that the mass of the gas in these galaxies is approximately 1 billion times that of the Sun.