Astronomers Spot a ‘Ghost Galaxy’ Made Almost Entirely of Dark Matter

In a crowded cluster of galaxies about 300 million light-years away, astronomers have confirmed the existence of a galaxy that is almost not there.

The object, known as Candidate Dark Galaxy-2, or CDG-2, emits roughly the light of a few million suns and is so diffuse that its glow barely rises above the background of space. Yet new observations with the Hubble Space Telescope, the European Space Agency’s Euclid mission and Japan’s Subaru Telescope suggest it is wrapped in a massive halo of invisible material. More than 99% of its mass appears to be dark matter.

The findings, published in 2025 in The Astrophysical Journal Letters and highlighted in a Feb. 18, 2026, release from ESA’s Hubble team, make CDG-2 one of the most dark-matter-dominated galaxies ever identified. The work offers an unusually stark test of how galaxies form and evolve inside their dark-matter cocoons—and a preview of how Euclid may uncover similar “ghost galaxies” across the sky.

“The elusive object dubbed CDG-2 may be composed of 99% dark matter,” the ESA/Hubble release said, calling it “among the most heavily dark matter-dominated galaxies ever discovered.”

A galaxy found by its star clusters

Unlike typical dwarf galaxies, which contain tens or hundreds of millions of stars, CDG-2 appears to host only a few million suns’ worth of starlight in total. The team estimates that its stars shine with about the luminosity of 6 million suns, though with large uncertainty. For public communication, ESA notes that CDG-2 “dimly shines with the light of only about 1 million Suns.”

Most of that light is bound up in just four compact globular star clusters—dense, spherical swarms of ancient stars that orbit galaxies like bees around a hive. The Milky Way hosts more than 150 such clusters. CDG-2, by contrast, seems to have only four, and those clusters contain an unusually large share of the galaxy’s total starlight.

“This is the first galaxy detected solely through its globular cluster population,” lead author Dayi (David) Li of the University of Toronto said in the ESA statement.

Li and colleagues first flagged CDG-2 in 2022 while mining Hubble images of the Perseus galaxy cluster (Abell 426). Using a statistical technique called a log-Gaussian Cox process, the team searched for small clumps of globular clusters that might reveal faint or invisible host galaxies.

In Perseus, they found an anomaly: four globular clusters packed unusually close together with no detectable galaxy around them. Simulations suggested it was highly unlikely for such a tight grouping to occur by chance among the cluster’s intergalactic star clusters. The team labeled the object CDG-2, a candidate “dark galaxy” with stars so sparse they could not yet be seen.

Euclid and Subaru reveal a faint smear of starlight

To confirm whether a galaxy was really there, Li’s group turned to new data from Euclid, launched by ESA in 2023 to map dark matter and dark energy, and wide-field images from Subaru’s Hyper Suprime-Cam in Hawaii. Combined with deeper analysis of the original Hubble images, the datasets revealed an extremely faint smear of diffuse light surrounding the four clusters—the ghostly glow of a galaxy.

The Euclid observations were decisive, said co-author Francine Marleau of the University of Innsbruck, who works on low-surface-brightness galaxies and Euclid science.

“The Euclid data clearly confirm the presence of the extremely faint, diffuse light of CDG-2, revealing the galaxy behind the globular clusters for the first time,” Marleau said in the ESA release.

How do you weigh a galaxy you can barely see?

Seeing the galaxy was only the first step. Weighing it—and separating its visible matter from its invisible component—required an indirect approach.

In many galaxies, astronomers infer dark matter by measuring how fast stars or gas clouds move and comparing that to the pull of gravity expected from visible material alone. Spiral galaxies, for example, rotate too quickly at their outskirts unless they are embedded in extended halos of unseen mass. In massive clusters like Perseus, the bending of light from background galaxies, known as gravitational lensing, also reveals large amounts of dark matter.

CDG-2 is far too faint and too distant for detailed measurements of internal stellar motions. Instead, Li’s team relied on an empirical relationship established in previous work: the total mass of a galaxy’s globular cluster system tends to scale with the mass of its dark-matter halo. By counting CDG-2’s globular clusters and estimating their total brightness, the researchers could infer how massive its surrounding dark halo must be.

They then compared that figure to the stellar mass implied by the galaxy’s feeble diffuse light. The result was striking. Under conservative assumptions, they concluded that dark matter accounts for at least 99.94% to 99.98% of CDG-2’s total mass. If CDG-2 hosts a typical, but partially unseen, population of fainter globular clusters, the dark-matter fraction could exceed 99.99%.

In their paper, the authors write that CDG-2 “may be the most GC dominated galaxy and potentially one of the most dark matter dominated galaxies ever discovered.”

Astronomers caution that the exact numbers depend on statistical relations that have built-in scatter and on assumptions about how many globular clusters remain undetected. The stellar mass itself also has uncertainties, because estimating how much mass is locked up in faint, old stars requires models of how bright those stars should be. No direct dynamical measurement of CDG-2’s mass—such as velocity readings for its star clusters—has yet been made.

An extreme case among ultra-diffuse galaxies

Even so, the evidence places CDG-2 at the extreme dark-matter-rich end of a class of puzzling objects known as ultra-diffuse galaxies. First identified in large numbers in the last decade, including with the Dragonfly Telephoto Array, these systems can be as large as the Milky Way but hundreds or thousands of times fainter. Some, like the Coma cluster galaxy Dragonfly 44, were initially reported to be extraordinarily dark-matter dominated, though those early estimates have since been revised.

At the opposite extreme, a handful of ultra-diffuse galaxies appear to be strangely deficient in dark matter. Two of the most prominent examples, NGC 1052-DF2 and NGC 1052-DF4, have internal stellar motions that seem to match the mass of their stars alone. Those galaxies were reported by a team that included Pieter van Dokkum and Shany Danieli of Yale University, both co-authors on the CDG-2 study.

The growing catalog of such outliers—galaxies with far more or far less dark matter than expected—has forced theorists to refine models of how galaxies interact with their environments and with dark matter.

A harsh environment, and a clue to dark matter’s behavior

CDG-2 orbits in one of the most hostile environments a small galaxy can inhabit. The Perseus cluster is filled with hot, X-ray-emitting gas and dominated by massive galaxies whose gravity can tear smaller systems apart. Many astronomers suspect CDG-2 began its life as a more familiar gas-rich dwarf galaxy that fell into Perseus and was gradually stripped.

As it plunged through the cluster, the pressure of the hot gas could have blown away its own gas, shutting off new star formation—a process known as ram-pressure stripping. Repeated gravitational encounters with larger galaxies and the cluster’s overall tidal field might have removed some of its outer stars and puffed up the rest, leaving behind a diffuse, nearly invisible stellar envelope. Globular clusters, which are tightly bound, and the dark-matter halo, which extends much farther than the starlight, would be more resistant to disruption.

That survival has implications for the properties of dark matter itself. To keep its star clusters intact under the stresses of the cluster environment, CDG-2’s dark-matter halo must be dense enough in its inner regions. The galaxy therefore offers a data point for theories that propose self-interacting or otherwise nonstandard dark matter, which can alter halo density profiles.

A preview of Euclid’s hidden-galaxy haul

CDG-2 is also an early demonstration of what Euclid can do beyond its main mission of tracing dark matter statistically across billions of light-years. By combining Euclid’s wide, low-surface-brightness imaging with Hubble’s sharp resolution and Subaru’s panoramic view, astronomers are beginning to expose a hidden population of almost-dark galaxies in nearby clusters.

If CDG-2 is any indication, many of those galaxies will be difficult to find. In most telescopes, they will show up not as obvious smudges of light but as small groupings of ancient star clusters—breadcrumbs leading to massive, invisible structures.

As Euclid continues to survey the sky and future observatories, including NASA’s Nancy Grace Roman Space Telescope, come online, researchers expect many more such systems to emerge. Each will offer another chance to watch how gravity, gas and dark matter have sculpted galaxies that, until now, barely registered in the cosmic census.