Astronomers have recently found 854 mysterious 'ultra-dark' galaxies in the Coma Cluster located about 321 light years away from Earth.

These galaxies are dark because they have one thousandth of the number of stars that a galaxy like our Milky Way has. The 'dark' galaxies are fluffy and still manage to hold together, hinting towards mysterious unknown force that is binding them.

This surprising discovery was made by a group of researchers from the Stony Brook University (the State University of New York) and the National Astronomical Observatory of Japan by analyzing archival data from the Subaru Telescope.

Nearly 47 of these galaxies were found in 2014, but this recent discovery has sent scientists scrambling for explanations.

Jin Koda, principal investigator of the study, said, "Not only these galaxies appear very diffuse, but they are very likely enveloped by something very massive".

Something invisible must be protecting the fragile star systems of these galaxies, said experts. They said that it could be something with a high mass, or it could be very likely an excessive amount of dark matter.

The component of visible matter, such as stars, is calculated to contribute only 1% or less to the total mass of each galaxy. The rest, i. e. dark matter, accounts for more than 99%.

The Subaru Telescope, with its large-aperture and wide-field camera, used under excellent seeing conditions, revealed that these dark galaxies contain old stellar populations. It also showed a spatial distribution similar to those of other brighter galaxies in the Coma Cluster.

This suggested that they have been a long-lived population of galaxies within the cluster. The amount of visible matter they contain, is less than 1%, and is extremely low as compared to the average fraction within the universe.