TORONTO, ON — June 16, 2025 — A University of Toronto team has uncovered tantalizing evidence of a ghostly galaxy that eluded telescopes until now. Candidate Dark Galaxy (CDG-2) — tucked inside the Perseus Galaxy Cluster, some 240 million light-years away — revealed itself through an unusually tight clump of four globular star clusters.
“Globular clusters don’t clump together unless there’s a substantial gravitational force — such as that from a galaxy — to hold them in place,” says Dayi Li, a PhD candidate in the Department of Statistical Sciences (DOSS) who led the discovery while developing new astrostatistical tools.
Li’s hierarchical Bayesian model treats the globular clusters in the sky as a spatial point process, teasing out faint galaxy signals from the positions of visible globular clusters. The approach can simultaneously infer a hidden galaxy’s location, size and globular cluster properties — and in testing the code, Li stumbled on CDG-2. "The probability that CDG-2 is a false positive — that is, that the tight clump of four globular clusters in CDG-2 arises purely by chance — is about 1 in 100,000," added Li. "This provides strong statistical evidence that CDG-2 might be an extremely dark galaxy."
Archival imaging from the Hubble Space Telescope and more recent imaging from ESA's Euclid mission exposed an almost imperceptible halo of faint starlight around the globular clusters, providing further physical evidence that CDG-2 is indeed a galaxy. To the researcher's knowledge, the discovery of CDG-2 marks the very first time a galaxy has been detected solely through its globular clusters.
Image: Euclid colour composite with Hubble inset (orange circle marks CDG-2; blue circles mark globular clusters).
CDG-2 is extreme: preliminary analysis suggests 99.99 percent of its mass is dark matter, while up to a third of its visible light may come from the globular clusters themselves. “This result opens a new window on galaxies that form inside nearly pure dark-matter haloes,” Li notes.
Li’s colleagues are equally enthusiastic:
- Prof. Gwendolyn Eadie praised the discovery, calling it a “truly interdisciplinary and excellent” piece of work. She added, “As one of the first CANSSI-Ontario Multidisciplinary Graduate Students, we couldn’t be more excited about Dayi’s research and this amazing discovery of CDG-2”.
- Prof. Roberto Abraham likens CDG-2 to “a needle in a cosmic haystack” found by “identifying the gravitational footprints galaxies leave behind.”
- Prof. Patrick Brown says watching spatial statistics reveal a new galaxy is “awe-inspiring — a testament to the power of interdisciplinary research.”
The research is published in The Astrophysical Journal Letters (DOI: 10.3847/2041-8213/adddab).
This research was funded by the Data Sciences Institute (DSI) at the University of Toronto and the multidisciplinary doctoral trainee program at CANSSI Ontario, as well as NSERC Discovery Grants to Li's supervisors.
About the Department of Statistical Sciences (DoSS), University of Toronto
Housed in the Faculty of Arts & Science, DoSS is Canada’s hub for cutting-edge research and education in statistics, data science and actuarial science. Its faculty, students and partners create new methodology and apply rigorous quantitative thinking to challenges ranging from health and finance to climate and astrophysics.