VIRGOHI21 is a dark matter halo in the Virgo cluster. It is the size of a galaxy, but apparently contains no stars. It was detected through H-I emissions of neutral hydrogen (21 cm emissions) in the dark galaxy. This is the first discovery of the postulated dark matter clumps and galaxies anticipated by dark-matter theories.
The galaxy has a mass of 100 million solar masses, and is 50 million light years away. The hydrogen gas present can only account for one thousandth of the mass detected, which has been estimated by taking into account the rate of rotation of the dark matter halo.
Other forms of dark galaxies are neutral hydrogen, with little dark matter and few or no stars. Consequently, these have very little mass, and are hardly anything more than a cloud of diffuse hydrogen gas. Dark matter clump galaxies, while having significantly more mass, have little or no baryonic matter ("normal" matter) and consequently few or no stars.
Though other such dark matter galaxies were thought to have been found in the past, this is the first one to conclusively have no stars in it. The relative abundance of these dark-matter halos is still being disputed, with some postulating that there could be more dark matter "galaxies" than regular ones.
Dark matter accounts for the majority of the universe's mass, and is present in "normal" galaxies as well. Indeed, the rotation and gluing together of normal galaxies is what led to the postulation of such dark matter. However, this galaxy appears to have a substantially higher percentage of dark matter than any galaxy discovered before. It is currently thought by most cosmologists that all galaxies came to be by clumps of dark matter condensing after the big bang, and consequently attracting hydrogen by the dark matter's gravitational attraction. Dark matter halos are different than our galaxy in that they failed to attract enough hydrogen to begin star birth.
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