VIRGOHI21 is an extended region of neutral hydrogen (HI) in the Virgo cluster discovered in 2005. Analysis of its internal motion indicates that it may contain a large amount of dark matter, as much as a small galaxy. Since VIRGOHI21 apparently contains no stars, this would make it one of the first detected dark galaxies. Skeptics of this interpretation argue that VIRGOHI21 is simply a tidal tail of the nearby galaxy NGC 4254.
Read more about VIRGOHI21: Observational Properties, Dark Galaxy Interpretation, Tidal Tail Interpretation