854 Ultra-dark galaxies discovered.

• Position: Astronomy

Eight-hundred fifty-four "ultra-dark galaxies" in the Coma Cluster have been discovered by a team of researchers from Stony Brook (N.Y.) University and the National Astronomical Observatory of Japan analyzing data from the 8.2-meter Subaru Telescope. The findings surpass the 2014 discovery of 47 mysterious dark galaxies and suggests that clusters are the key environment for the evolution of these mysterious dark galaxies.

"The findings suggests that these galaxies appear very diffuse and are very likely enveloped by something very massive," says principal investigator Jin Koda, associate professor in the Department of Physics and Astronomy at Stony Brook.

The ultra-dark galaxies are similar in size to the Milky Way, but have only 1/1,000 of stars that our galaxy does. The stellar population within such fluffy extended galaxies is subject to rapid disruption due to a strong tidal force detected within the cluster.

"We believe that something invisible must be protecting the fragile star systems of these galaxies, something with a high mass. That 'something' is very likely an excessive amount of dark matter," explains Koda. "The component of visible matter, such as stars, is calculated to contribute only one percent or less to the total mass of each galaxy. The rest--dark matter--accounts for more than 99%.

The Subaru Telescope revealed that these dark galaxies contain old stellar populations and shows a spatial distribution similar to those of other brighter galaxies in the Coma Cluster. It suggests that there has been a...