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An ultra diffuse galaxy (UDG) is an extremely-low-density galaxy first discovered in the nearby Virgo Cluster by Allan Sandage and Bruno Binggeli in 1984. Such a galaxy may have the same size as the Milky Way but a visible star count of only 1%. Their lack of luminosity is due to the lack of star-forming gas in the galaxy. This results in old stellar populations.[1][2]

Some UDGs found in the Coma Cluster, about 330 million light years from Earth, have diameters of 60 kly (18 kpc) (more than half the size of our galaxy) with 1% of the stars of the Milky Way Galaxy.[3] The distribution of ultra diffuse galaxies in the Coma Cluster is the same as luminous galaxies; this suggests that the cluster environment strips the gas from the galaxies, while allowing them to populate the cluster the same as more luminous galaxies. The similar distribution in the higher tidal force zones suggests a larger dark matter fraction to hold the galaxies together under the higher stress.[1]

Dragonfly 44,[2] a UDG in the Coma Cluster, is one example. Observations of the rotational speed suggest a mass of about one trillion solar masses, about the same as the mass of the Milky Way. This is also consistent with about 90 globular clusters observed around Dragonfly 44. However, the galaxy emits only 1% of the light emitted by the Milky Way.[4] On 25 August 2016, astronomers reported that Dragonfly 44 may be made almost entirely of dark matter.[5][6][7]

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  1. ^ a b "Astronomers discover 854 ultra-dark galaxies in the famous Coma Cluster". Science Daily. 22 June 2015. 
  2. ^ a b "Scientists discover the fluffiest galaxies". 14 May 2015. 
  3. ^ "Scientists at Keck Discover the Fluffiest Galaxies". Space Daily. 18 May 2015. 
  4. ^ Crosswell, Ken (26 July 2016). "The Milky Way's dark twin revealed". Nature News. Retrieved 30 July 2016. 
  5. ^ Van Dokkum, Pieter; et al. (25 August 2016). "A High Stellar Velocity Dispersion and ~100 Globular Clusters For The Ultra-Diffuse Galaxy Dragonfly 44". The Astrophysical Journal Letters. 828: L6. arXiv:1606.06291 . Bibcode:2016ApJ...828L...6V. doi:10.3847/2041-8205/828/1/L6. Retrieved 27 August 2016. 
  6. ^ Hall, Shannon (25 August 2016). "Ghost galaxy is 99.99 per cent dark matter with almost no stars". New Scientist. Retrieved 27 August 2016. 
  7. ^ Feltman, Rachael (26 August 2016). "A new class of galaxy has been discovered, one made almost entirely of dark matter". The Washington Post. Retrieved 26 August 2016.