Hubble Reveals CDG-2, a Galaxy 99% Dark Matter

Astronomers using Hubble, with Euclid and Subaru, identified CDG-2 in the Perseus cluster as an extremely dark-matter–dominated galaxy—about 245 million to 300 million light-years away—where 99% of its mass is dark matter. The galaxy was detected indirectly by tracing four tightly packed globular clusters, which reveal a faint glow around them. Its normal matter is estimated at roughly 6 million sun-like stars, with about 16% accounted for by the globular clusters, suggesting past star formation that has since been stripped away by gravitational interactions. The discovery, highlighting a “dark galaxy” category, was reported in The Astrophysical Journal Letters.