Ultra dark galaxies appear very diffuse and remarkably extended as seen by the light of the stars they contain. Many are similar in size to the Milky Way, but 1,000 fainter than our Galaxy. Astronomers think these mysterious dark galaxies contain excessive amount of dark matter, and thus may offer another insight into galaxy formation model. In 2015, the Subaru Telescope discovered more than 800 ultra dark galaxies in the Coma Cluster, suggesting galaxy clusters as the key environment for the evolution of ultra dark galaxies. Astronomers also revealed stellar populations in these galaxies. Dark matter is invisible, but measurements of stellar motions are expected to expose the distribution of dark matter in these dark galaxies.