Astronomers have detected a ghostly galaxy that is almost completely made of dark matter. The faint object was revealed by just four globular star clusters hiding in the Perseus cluster.
While most galaxies blaze with billions of stars, a rare few are so faint they are barely detectable. These low-surface-brightness galaxies contain very few stars and are largely made up of dark matter, making them extremely difficult to spot.
One of the most unusual examples, called CDG-2, could be among the most dark matter dominated galaxies ever identified. (Dark matter is an invisible form of matter that does not reflect, emit, or absorb light.) The discovery was detailed in The Astrophysical Journal Letters.
Finding a Galaxy Through Globular Clusters
Because these galaxies emit so little light, they are challenging to detect directly. David Li of the University of Toronto, Canada, and his research team used sophisticated statistical methods to search for patterns instead of brightness. They looked for tight groupings of globular clusters, which are dense, spherical collections of stars that usually orbit larger galaxies. Such clusters can serve as indirect evidence that a faint galaxy is present.
Using this strategy, the team located 10 already known low-surface-brightness galaxies and identified two additional candidates that may qualify as dark galaxies.
Confirming CDG-2 With Hubble, Euclid, and Subaru
To confirm one of the candidates, astronomers relied on three observatories: NASA’s Hubble Space Telescope, ESA’s (European Space Agency) Euclid space observatory, and the Subaru Telescope in Hawaii. High-resolution images from Hubble revealed four tightly grouped globular clusters within the Perseus galaxy cluster, about 300 million light-years from Earth.
Further observations combining data from Hubble, Euclid, and Subaru detected a faint, extended glow surrounding those clusters. That dim halo provided convincing evidence that a previously unseen galaxy lay beneath them.
“This is the first galaxy detected solely through its globular cluster population,” said Li. “Under conservative assumptions, the four clusters represent the entire globular cluster population of CDG-2.”
A Galaxy That Is 99 Percent Dark Matter
Early estimates suggest CDG-2 shines with the combined light of roughly 6 million Sun-like stars. The four globular clusters account for about 16% of the galaxy’s visible light. Even more striking, about 99% of its total mass, including both visible matter and dark matter, appears to be dark matter.
Most of the ordinary matter needed to form stars, mainly hydrogen gas, was likely stripped away through gravitational interactions with neighboring galaxies inside the Perseus cluster.
Globular clusters are extremely dense and strongly bound by gravity. Their resilience makes them less vulnerable to gravitational tidal disruption, allowing them to remain intact and act as dependable markers of faint galaxies like CDG-2.
Expanding the Search for Dark Matter Galaxies
As major sky surveys continue to grow with missions such as Euclid, NASA’s upcoming Nancy Grace Roman Space Telescope, and the Vera C. Rubin Observatory, astronomers are increasingly using machine learning and advanced statistical techniques to analyze enormous datasets.
The Hubble Space Telescope has operated for more than 30 years and continues to deliver discoveries that deepen our understanding of the universe. Hubble is a joint project between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, oversees mission operations, with additional support from Lockheed Martin Space in Denver. The Space Telescope Science Institute in Baltimore, operated by the Association of Universities for Research in Astronomy, manages Hubble’s science operations for NASA.
Reference: “Candidate Dark Galaxy-2: Validation and Analysis of an Almost Dark Galaxy in the Perseus Cluster” by Dayi (David) Li, Qing Liu, Gwendolyn M. Eadie, Roberto G. Abraham, Francine R. Marleau, William E. Harris, Pieter van Dokkum, Aaron J. Romanowsky, Shany Danieli, Patrick E. Brown and Alex Stringer, 16 June 2025, The Astrophysical Journal Letters.
DOI: 10.3847/2041-8213/adddab
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4 Comments
Dark Matter is beginning to look like the 21st century “Either” problem.
There is no such thing as “dark matter”.
It’s the usual problem with dark matter.. always very far away, never any around here. No one knows anything about dark matter due to it always being very far away!
In response to previous comments, my layman understanding is that the term “dark matter” is more of a moniker or place holder for something that is definitely exerting a gravitational (or gravitational-like?) effect on “regular” matter in the universe. We don’t understand its nature but we have to call it something! This is the same with dark energy to the best of my understanding.