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Phoenix666 writes:

About 321 million light-years away from us is the Coma Cluster, a massive grouping of more than 1,000 galaxies. Some of its galaxies are a little unusual, however: they're incredibly dim. So dim, in fact, that they have earned the title of "Ultra-Dark Galaxies" (UDGs). (The term is actually "Ultra-Diffuse Galaxies", as their visible matter is thinly spread, though "ultra-dark" has been used by some sources and, let's face it, sounds a lot better). This was discovered earlier this year in a study that identified 47 such galaxies.

Dimness isn't necessarily unusual in a galaxy. Most of a galaxy's light comes from its stars, so the smaller a galaxy is (and thus the fewer stars it has), the dimmer it will be. We've found many dwarf galaxies that are significantly dimmer than their larger cousins.

What was so unusual about these 47 is that they're not small enough to account for their dimness. In fact, many of them are roughly the size of our own Milky Way (ranging in diameter from 1.5 to 4.6 kiloparsecs, compared with the Milky Way's roughly 3.6) but have only roughly one thousandth of the Milky Way's stars. The authors of the recent study interpret this to mean that these galaxies must be even more dominated by dark matter than are ordinary galaxies.