Candidate Dark Galaxy 2 (CDG-2) is a dark galaxy composed predominantly of dark matter, along with four globular clusters, in the Perseus cluster.[1] It is one of the most dark matter dominated galaxies ever discovered, estimated to be between 99.94% and 99.98% dark matter.[2]
CDG-2 was discovered by a Hubble Space Telescope survey (the PIPER survey) looking for larger than expected densities of globular clusters in the Perseus cluster. These higher densities of globular clusters could correspond to ultra-diffuse galaxies. In March 2025, researchers used a new statistical technique, a Poisson cluster process (adapted from the Neyman–Scott process), to perform the search and identified CDG-2 as a possible galaxy.[3] In June 2025 the Hubble Space Telescope images, images from the Euclid survey, and images from the Subaru telescope were examined[4] and diffuse emission around the globular clusters in CDG-2 was found, providing strong evidence that CDG-2 was indeed a galaxy. It is the first galaxy to be detected through the presence of globular clusters.[5]
Unlike most ultra-diffuse galaxies that have been discovered so far, CDG-2 is believed to only have the four globular clusters originally identified, though there were a few possible additional globular clusters that required further investigation. CDG-2 is estimated at having a dark matter halo mass of approximately , meaning that it is a highly dark matter dominated galaxy, with a halo mass fraction of 99.94 to 99.98%.[5] The existence of this galaxy may be a useful test for various theories of galaxy formation.[6] Since dark galaxies are a prediction of the Lambda-CDM model, the prevalence of dark galaxies is an important test of the theory, and simulations of their prevalence have drawn upon CDG-2 as a reference, along with Cloud-9.[7]
