Hubble Finds One of the Darkest Known Galaxies

CDG-2 is a low-surface-brightness galaxy dominated by dark matter and containing only a sparse scattering of faint stars. Preliminary analysis suggests it has the luminosity of roughly 6 million Sun-like stars, with its globular clusters making up about 16% of that visible content; roughly 99% of its mass appears to be dark matter.

Researchers first flagged CDG-2 by searching for tight groupings of globular clusters, compact spherical star groups that often orbit normal galaxies and can reveal hidden stellar populations. Using Hubble, ESA’s Euclid observatory, and the Subaru Telescope, astronomers found a close collection of four globular clusters in the Perseus galaxy cluster and then detected a faint, diffuse glow surrounding them — strong evidence of an underlying galaxy.

“This is the first galaxy detected solely through its globular cluster population,” said Li.

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