https://www.reddit.com/r/space/comments/1rabziu/a_galaxy_composed_almost_entirely_of_dark_matter/
Astronomers using Hubble, ESA’s Euclid and Japan’s Subaru telescope have found a galaxy that is barely there. Candidate Dark Galaxy-2 (CDG-2) sits in the Perseus cluster about 300 million light-years away. It was spotted as a tight group of four globular star clusters, then confirmed by a very faint glow around them.
It shines with roughly 6 million Suns’ worth of light, and the four clusters provide about a sixth of that. NASA says about 99% of its mass appears to be dark matter. The paper’s estimates go to 99.94% or higher, depending on assumptions. Its gas was probably stripped away by other galaxies in the cluster.
It’s still a candidate. Spectroscopy is needed to nail it down.
Source: www.reddit.com · Also: NASA · ApJL paper · ESA/Hubble