Name: Anonymous 2010-02-01 15:47
Virgo H121 is an entire galaxy made of dark matter. As quoted from page 33 in the December issue of Discover Magazine,
"Dark Galaxy
Name: Virgo H121
Location: Virgo cluster, 50 million light-years from Earth
Age: Presumably the same as the Milky Way
General Characteristics Virgo H121 is a galaxy trapped in perpetual darkness. In fact, it confounds all understanding of what a galaxy is. Galaxies are supposed to have stars, but this one has none. Almost by definition, galaxies are supposed to be visible objects, but this one can be seen only by radio-wave detectors. That is the only way that Robert Minchin, then at Cardiff University in Wales, and his colleagues were able to locate it--by tuning in to its feeble radio emissions
These emissions identify Virgo H121 as a swirling cloud of hydrogen-rich gas containing 100 million times the mass of the sun, making it larger than many dwarf visible galaxies. The speed of the cloud's rotation indicates its total mass, which leads to another surprise. In addition to the hydrogen, the galaxy is chock-full of some other form of mass that emits nothing at all, not even radio waves. Astronomers use the generic term dark matter to describe material that seems to make up the bulk of the mass of most galaxies, including our own. The nature of the dark matter is unknown. Virgo H121 is thus a double enigma: a galaxy whose ordinary matter is dark and whose bulk is dominated by a second, even darker component.
Johnathan Davies of Cardiff, one of the codiscoverers of Virgo H121, suggests that this object is merely at the extreme end of the range of galaxy evolution. Other astronomers have found dim galaxies that seem to form stars only very slowly. Virgo H121 has such a low density that it apparently never reached the point at which gravity begins to pull gas clouds together as stars.
Range & Population This is the only completely dark galaxy discovered, but it is unlikely to be unique. Some computer simulations of cosmic structure imply that similar small, but dark galaxies may outnumber the bright ones as much as 100 to 1.
How to spot it Ask for time on a radio telescope, or content yourself with bright galaxies instead." - Corey S. Powell