Galaxy Made up of 99% Dark Matter Discovered by Hubble

Why It Matters

Revealing a galaxy that is 99% dark matter reshapes our understanding of hidden mass structures and informs future dark‑matter and galaxy‑formation research.

Key Takeaways

• CDG2 is a low‑surface‑brightness galaxy in Perseus cluster
• Dark matter comprises roughly 99% of CDG2’s total mass
• Discovery relied on globular cluster density, not galaxy’s light
• Hubble, Subaru, and ISA’s Uklid confirmed faint diffuse halo
• Stripped hydrogen gas explains low luminosity, clusters remain intact

Summary

Astronomers using Hubble and partner observatories have identified CDG2, an ultra‑faint, low‑surface‑brightness galaxy embedded in the Perseus cluster, whose visible glow is comparable to only six million suns.

The team inferred the galaxy’s presence by a subtle rise in the density of ancient globular clusters, which orbit a massive unseen mass. Analysis shows the clusters account for about 16% of CDG2’s detectable matter, while dark matter makes up roughly 99% of its total mass, indicating extreme dark‑matter domination.

Lead researcher described the detection as “spotting a black cat in a coal bin,” emphasizing the challenge. Hubble, the Subaru Telescope, and the ISA’s Uklid Observatory captured a faint halo of diffuse light surrounding four of the clusters, confirming the galaxy’s existence.

The find expands the census of dark‑matter‑rich galaxies, suggesting many similar “ghost” systems may lurk undetected, and provides a new laboratory for testing galaxy‑formation models and future missions like the Roman Space Telescope.

Original Description

The Hubble Space Telescope has discovered a galaxy in the the Perseus galaxy cluster that is "composed of 99% dark matter," according to NASA's Goddard Space Flight Center.

Credit: NASA's Goddard Space Flight Center

Paul Morris: Lead Producer

Video Credits:

A simulation of the formation of dark matter structures from the early universe until today.

Ralf Kaehler/SLAC National Accelerator Laboratory, American Museum of Natural History