from Nature 523, 9 (02 July 2015) doi:10.1038/523009b
Astronomers have discovered more than 850 faint galaxies in a galaxy cluster that could be made mostly of dark matter.
Using archived images from the Subaru Telescope in Hawaii, a team led by Jin Koda at Stony Brook University in New York searched for observations of the Coma galaxy cluster, which is roughly 101 million parsecs (330 million light years) away. The team found 854 ultra-diffuse galaxies, a class of faint galaxy that can be as large as the Milky Way, but which has only 0.1% the number of stars. For these galaxies to remain gravitationally bound together, the researchers show that more than 99% of their mass must be dark matter.
This suggests that the crowded environment sucks gas away from these galaxies, leaving them largely unable to form stars.
Paper: Approximately a thousand ultra-diffuse galaxies in the coma cluster by Jin Koda, Masafumi Yagi, Hitomi Yamanoi, and Yutaka Komiyama