Apparently the Virgo Cluster has some even stranger objects lurking there than we've thought:

http://www.space.com/scienceastronomy/050223_dark_galaxy.html

They discovered a radio source that is too massive for its spin rate, so they think it must be a "dark galaxy", embedded in a halo of dark matter.

If they're right about that, they think there must be many thousands more like it awaiting detection.

Yea, I saw that today on CNN. It's an interesting concept, but wouldn't they have to alter the definition of galaxy in order to include the structure? What I mean is, there would have to be stars there made completely out of dark matter as well as globular's, nebulae and all of the other sort of objects generally associated with galactic structure.

I realize it is a new discovery and so this will likely change in the future.