According to our current inventory of the universe, there is a great deal more dark matter than ordinary matter (where the dark matter is made of some particle that has never yet been observed in the laboratory). Indeed, the lovely pictures we see of galaxies should be interpreted as sprinkles of shining gas and stars settled into the lower reaches of large, extended halos of dark matter.
Of course, this idea suggests the possibility that there might be galaxies which are almost entirely made of dark matter, with very few stars at all. You might even expect that such objects should exist; there are various ways, for example, that some sort of shock wave or galactic collision could remove most of the ordinary matter from a galaxy while leaving the dark matter intact. And now there is a claim that a dark galaxy has been found. Here is the astro-ph paper being referred to in the news article. (Someday, people will understand the internet so well that they will link news stories directly to the technical papers.)
How do you find a dark galaxy? In principle you could do it by looking purely at its gravitational field, for example through gravitational lensing of background galaxies. In this case, however, the astronomers are simply using the fact that the galaxy is not 100% dark — it contains neutral hydrogen that can be detected by radio observations. But it’s not very much hydrogen; perhaps 108 solar masses, while the galaxy as a whole comes in at 1011 solar masses.
If it’s true, the existence of this galaxy would have important consequences for models of galaxy formation in the presence of dark matter. Even better, it would make it very hard to maintain that there might not be any dark matter, and instead gravity is modified on galactic scales (as in the MOND model). It’s too early to jump to conclusions, though. A skeptical note has been sounded by my erstwhile graduate-school office mate, Mike Merrifield, who points out that a chance superposition of two different small hydrogen clouds could trick you into thinking that there was one big rapidly-rotating cloud. But the observers will keep looking, gradually piecing together the ingredients of this preposterous universe.