Hubble, Euclid, and Subaru Discover a Galaxy That Is 99% Dark Matter—Detected Only by Its Globular Clusters
CDG-2, a near-invisible galaxy 300 million light-years away, is the first detected solely through its globular cluster population. Roughly 99% of its mass is dark matter.
Overview
A team led by David Li at the University of Toronto has identified one of the most extreme galaxies ever found. Designated CDG-2 (Candidate Dark Galaxy-2), it sits roughly 300 million light-years away within the Perseus galaxy cluster and appears to consist of approximately 99% dark matter—making it nearly undetectable by conventional observation. The findings, published in The Astrophysical Journal Letters, were confirmed using a combination of NASA’s Hubble Space Telescope, ESA’s Euclid observatory, and the Subaru Telescope in Hawaii.
What We Know
CDG-2 belongs to a class of objects called low-surface-brightness galaxies—structures so diffuse and faint that they are almost invisible against the background sky. According to NASA Science, the galaxy was not detected through its stars at all. Li’s team discovered it by hunting for tight groupings of globular clusters, the compact spherical star systems that serve as tracers of otherwise invisible galactic structure.
“This is the first galaxy detected solely through its globular cluster population,” Li stated, as quoted in the ESA Hubble press release.
The result is a galaxy with only four confirmed globular clusters—compared to the more than 150 orbiting the Milky Way—and a total luminosity equivalent to roughly one million Sun-like stars. As described by Space.com, the four clusters account for approximately 16% of CDG-2’s visible content, with normal stellar matter making up almost none of the remainder. The vast majority of the galaxy’s mass is gravitationally inferred rather than directly observed.
The leading explanation for CDG-2’s unusual composition is gravitational stripping. Within dense galaxy clusters like Perseus, tidal forces from neighboring galaxies can tear away a galaxy’s supply of gas and ordinary matter—the raw material for star formation—while leaving dark matter largely intact, since dark matter responds only to gravity and is not dragged away by hydrodynamic effects. As reported by ScienceDaily, this process is thought to have left CDG-2 as a gravitational shell: structurally massive by dark matter content, but nearly barren of the luminous material that makes most galaxies visible.
Why It Matters
Dark matter accounts for roughly 27% of the universe’s total mass-energy content, yet its fundamental nature remains unknown. Galaxies at the extreme end of the dark matter spectrum—where visible matter has been almost entirely lost—offer a rare opportunity to probe how dark matter is distributed and held together independent of normal baryonic matter.
CDG-2 also demonstrates a methodological advance. Identifying galaxies by their globular cluster populations rather than their stellar light opens a new detection channel for objects that would otherwise remain entirely hidden. Li’s team first identified 10 confirmed low-surface-brightness galaxies and two additional dark galaxy candidates using the same statistical technique, suggesting CDG-2 may not be an isolated case.
What We Don’t Know
The precise ratio of dark to visible matter in CDG-2 rests on mass estimates derived from gravitational lensing and cluster dynamics, which carry inherent uncertainties. The total number of dark-matter-dominated galaxies in the Perseus cluster—and in the wider universe—remains unknown. Whether CDG-2 formed with minimal normal matter or was progressively stripped over billions of years has not been definitively established.