A newly identified candidate for a dark galaxy, designated Candidate Dark Galaxy-2 (CDG-2), is primarily composed of dark matter and exhibits minimal light emission. This celestial entity comprises four globular clusters and resides within the Perseus galaxy cluster. The emergence of CDG-2 introduces novel questions regarding galactic formation and evolution within the cosmic web, simultaneously offering an unprecedented perspective on the universe’s most enigmatic constituent: dark matter, which vastly exceeds ordinary matter in mass yet remains invisible.
CDG-2 (indicated by a dashed red circle) is predominantly dark matter and hosts a sparse distribution of stars. Image courtesy of NASA / ESA / Dayi Li, UToronto / Joseph DePasquale, STScI.
“Throughout the immense cosmic expanse, the majority of galaxies radiate intensely across vast stretches of time and space,” remarked David Li, an astronomer from the University of Toronto, alongside his research associates.
“However, a diminutive fraction of galaxies are virtually undetectable—these are low-surface-brightness galaxies, overwhelmingly constituted by dark matter and containing only a meager scattering of dim stars.”
“The detection of such faint galaxies presents a formidable challenge.”
Employing sophisticated statistical methodologies, Dr. Li and his collaborators pinpointed ten previously validated low-surface-brightness galaxies, along with two supplementary dark galaxy contenders, by meticulously searching for tightly aggregated globular clusters.
These aggregations can serve as indicators for the presence of an obscure, underlying stellar population.
To substantiate one of these dark galaxy candidates, observations were conducted utilizing the NASA/ESA Hubble Space Telescope, ESA’s Euclid space observatory, and the terrestrial Subaru Telescope situated in Hawaii.
Hubble’s high-resolution imagery unveiled a compact congregation of four globular clusters within the Perseus cluster, a substantial galaxy assemblage located approximately 240 million light-years distant in the Perseus constellation.
Subsequent investigations employing Hubble, Euclid, and Subaru subsequently revealed a faint, dispersed luminescence enveloping the stellar clusters—compelling evidence of an underlying galactic structure.
“This marks the inaugural instance of a galaxy being identified solely through its globular cluster population,” stated Dr. Li.
“Adopting conservative estimations, the four identified clusters represent the entirety of CDG-2’s globular cluster complement.”
Initial assessments indicate that CDG-2 possesses a luminosity equivalent to that of approximately 6 million Sun-like stars, with its globular clusters constituting 16% of its observable mass.
Remarkably, an overwhelming 99% of its total mass, encompassing both luminous and dark matter, appears to be dark matter.
A significant portion of its ordinary matter, which would normally facilitate star formation, was likely dislodged through gravitational encounters with other galaxies within the Perseus cluster.
“CDG-2 may represent the galaxy most dominated by globular clusters and potentially one of the most dark matter-dominated galaxies ever brought to light,” the researchers concluded.
Their publication appeared in June 2025 in the Astrophysical Journal Letters.
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Dayi (David) Li et al. 2025. Candidate Dark Galaxy-2: Validation and Analysis of an Almost Dark Galaxy in the Perseus Cluster. ApJL 986, L18; doi: 10.3847/2041-8213/adddab
