CDG-2

CDG-2
Image of Candidate Dark Galaxy 2 (CDG-2) taken by Hubble Space Telescope, with the four globular clusters in it highlighted.
Observation data
Constellation	Perseus
Right ascension	03h 17m 12.61s
Declination	+41° 20′ 52.64″
Distance	About 245 million lightyears
Group or cluster	Perseus
References: https://science.nasa.gov/asset/hubble/dark-galaxy-cdg-2-near-perseus-cluster/

Candidate Dark Galaxy 2 (CDG-2) is a dark galaxy composed predominantly of dark matter, along with four globular clusters in the Perseus cluster. ^[1] It is one of the most dark matter dominated galaxies ever discovered, estimated to be between 99.94% and 99.98% dark matter. ^[2]

Discovery

CDG-2 was discovered by a Hubble Space Telescope survey (the PIPER survey) looking for larger than expected densities of globular clusters in the Perseus cluster. These higher densities of globular clusters could correspond to ultra-diffuse galaxies. In March 2025, researchers used a new statistical technique, a Poisson cluster process (adapted from the Neyman–Scott process), to perform the search and identified CDG-2 as a possible galaxy. ^[3] In June 2025 the Hubble Space Telescope images, images from the Euclid survey, and images from the Subaru telescope were examined ^[4] and diffuse emission around the globular clusters in CDG-2 was found, providing strong evidence that CDG-2 was indeed a galaxy. It is the first galaxy to be detected through the presence of globular clusters. ^[5]

Contents

Unlike most ultra-diffuse galaxies that have been discovered so far, CDG-2 is believed to only have the four globular clusters originally identified, though there were a few possible additional globular clusters that required further investigation. CDG-2 is estimated at having a dark matter halo mass of approximately ${\displaystyle 5.7\times 10^{10}M_{\odot }}$, meaning that it is a highly dark matter dominated galaxy, with a halo mass fraction of 99.94 to 99.98%. ^[5] The existence of this galaxy may be a useful test for various theories of galaxy formation. ^[6]

References

1. "Dark Galaxy CDG-2 Near Perseus Cluster - NASA Science". 2026-02-18. Retrieved 2026-02-21.
2. published, Robert Lea (2026-02-19). "Hubble telescope discovers rare galaxy that is 99% dark matter". Space. Retrieved 2026-02-21.
3. Li, Dayi; Stringer, Alex; Eadie, Gwendolyn; Abraham, Roberto (March 2025). "Poisson cluster process models for detecting ultra-diffuse galaxies". Annals of Applied Statistics. 19 (1): 261–285 – via Project Euclid.
4. [e-mail]. "Hubble identifies one of darkest known galaxies - The elusive object dubbed CDG-2 may be composed of 99% dark matter". www.esahubble.org. Retrieved 2026-02-21.
5. Li 李, Dayi (David) 大一; Liu 刘, Qing 青; Eadie, Gwendolyn M.; Abraham, Roberto G.; Marleau, Francine R.; Harris, William E.; van Dokkum, Pieter; Romanowsky, Aaron J.; Danieli, Shany; Brown, Patrick E.; Stringer, Alex (2025-06-16). "Candidate Dark Galaxy-2: Validation and Analysis of an Almost Dark Galaxy in the Perseus Cluster". The Astrophysical Journal Letters. 986 (2): L18. doi: 10.3847/2041-8213/adddab . ISSN   2041-8205.
6. Garay, Jorge. "A Galaxy Composed Almost Entirely of Dark Matter Has Been Confirmed". Wired. ISSN   1059-1028 . Retrieved 2026-02-21.