Most galaxies are discovered because they contain enough stars to glow clearly in telescope images. CDG-2 was found in a far stranger way. In February 2026, NASA and ESA described it as one of the darkest known galaxies: an object in the Perseus Cluster that is so faint it may be made almost entirely of dark matter, with only a tiny sprinkling of stars. (science.nasa.gov)

The key clue was not starlight, but globular clusters. These are dense, tightly bound groups of old stars that often orbit galaxies. Dayi Li and his team searched Hubble survey data of the Perseus Cluster for unusually tight over-densities of globular clusters, using an advanced statistical model designed to detect hidden galaxies from the positions of those clusters alone. In that search, CDG-2 appeared as a close grouping of four globular clusters. (science.nasa.gov)

That was only the beginning. To test whether the cluster group really marked a galaxy, astronomers combined archival Hubble images with newer observations from ESA’s Euclid mission and additional data from the Subaru Telescope in Hawaiʻi. Together, these data revealed an extremely faint, diffuse glow around the four clusters. That weak halo of starlight was crucial: it showed that the clusters were not just a random grouping, but part of a real galaxy. According to the researchers, this is the first galaxy ever detected purely through its globular-cluster population. (science.nasa.gov)

CDG-2 is remarkable not only because it is dim, but because of what that dimness suggests. The team estimates that its total light is only about that of 6 million Suns, and at least about 16% of that visible light comes from the globular clusters themselves. If the usual relation between globular clusters and dark-matter halos holds here, then roughly 99.94% to 99.99% or more of CDG-2’s mass could be dark matter. In other words, astronomers may have found a galaxy by following a few ancient star clusters like breadcrumbs through the dark. (science.nasa.gov)