New evidence has been discovered by an international team led by astronomers from the National Science Foundation’s Arecibo Observatory and from Cardiff University in the United Kingdom that VIRGOHI 21, a mysterious cloud of hydrogen in the Virgo Cluster 50 million light-years from the Earth, is a Dark Galaxy, emitting no starlight.

Their results not only indicate the presence of a dark galaxy but also explain the long-standing mystery of its strangely stretched neighbour. Skeptics of the dark-matter interpretation argue that VIRGOHI21 is simply a tidal tail of the nearby galaxy NGC 4254.

The observations, made with the Westerbork Synthesis Radio Telescope in the Netherlands, show that the hydrogen gas in VIRGOHI 21 appears to be rotating, suggesting a dark galaxy with over ten billion times the mass of the Sun. Only one percent of this mass has been detected as neutral hydrogen – the rest appears to be dark matter.

The results may also solve a long-standing puzzle about nearby galaxy NGC 4254 -a lopsided object, with one spiral arm much larger than the rest. This is usually caused by the influence of a companion galaxy, but none could be found until now – the team thinks VIRGOHI 21 is the culprit.

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