Astronomers have identified a galaxy, designated CDG-2 (Candidate Dark Galaxy-2), composed of approximately 99.9 percent dark matter, making it one of the darkest known galaxies in the universe. Located roughly 300 million light-years away in the Perseus galaxy cluster, the galaxy is exceptionally faint, visible only through the light emitted by four globular clusters within it.
For years, these compact groups of stars were thought to be independent objects. However, a study published in The Astrophysical Journal Letters presents evidence that these globular clusters are gravitationally bound to a larger, dark matter-dominated structure. The research team, led by David Li of the University of Toronto, Canada, utilized data from the Hubble Space Telescope, ESA’s Euclid space observatory, and the Subaru Telescope in Hawaii to confirm this connection.
The combined observations revealed a faint glow surrounding the four globular clusters, indicating the presence of an underlying galaxy too dim to be detected by any of the telescopes individually. These clusters contribute approximately 16 percent of CDG-2’s total brightness. Preliminary analysis suggests the galaxy’s total luminosity is equivalent to about 6 million suns.
Dark matter, which does not interact with light, makes up an estimated 85 percent of the matter in the universe and roughly 27 percent of its total energy density. Its existence is inferred from its gravitational effects on visible matter and the structure of the cosmos. Although dark matter halos are understood to surround most galaxies, CDG-2 presents an extreme case, with an extraordinarily high proportion of dark matter relative to conventional matter.
Astronomers estimate that dark matter accounts for between 99.94 and 99.98 percent of CDG-2’s total mass. The discovery of such “dark galaxies” is rare and provides a unique opportunity to study the nature of dark matter and test current models of galaxy formation. A similar, though distinct, observation in the AS1063 galaxy cluster revealed tens of thousands of globular clusters tracing the distribution of dark matter, also exhibiting large central cores, as detailed in a recent paper published on arXiv.
Detecting these faint galaxies is a significant challenge. The research team employed advanced statistical techniques to identify CDG-2 by searching for tight groupings of globular clusters, which can signal the presence of a hidden stellar population. Further high-resolution hydrodynamical simulations, requiring several trillion particles and incorporating over a hundred thousand globular clusters, are needed to better understand the dynamics at play and potentially rule out standard dark matter models.
