A Galaxy Made Almost Entirely of Dark Matter Has Been Confirmed
Here’s the headline that challenges our intuitions: a galaxy so dim that nearly all of its mass is invisible to us. Astronomers now have solid evidence that a galaxy can be almost wholly composed of dark matter, with the visible stars making up just a whisper of its total mass.
The object in question lies roughly 300 million light-years away and appears almost entirely invisible to the naked eye. What stands out are four globular clusters—compact star groups that resemble tiny islands in a vast cosmic void. For years these clusters in the Perseus cluster were thought to be separate entities.
Dubbed Candidate Dark Galaxy-2 (CDG-2), this galaxy is detected only through these four bright clusters, which together account for about 16 percent of its overall luminosity. Scientists estimate that a staggering 99.9 percent of CDG-2’s mass is dark matter, with the remaining 0.1 percent being ordinary matter.
A comprehensive study published in The Astrophysical Journal Letters confirms that the four globular clusters are indeed part of the same galaxy dominated by dark matter. This makes CDG-2 the first galaxy observed primarily through the light of its brightest fragments rather than through a conventional, star-filled glow.
To reach these conclusions, researchers combined observations from three of the most powerful telescopes: Hubble, Euclid, and Subaru. Individually, each telescope saw only faint hints, but together they revealed a subtle halo of light surrounding the globular clusters—evidence of a galaxy so faint that the instruments on their own would miss it.
What the data reveal is a galaxy with total luminosity equivalent to about 6 million suns, with the four globular clusters contributing about 16 percent of that light—a notably high share for such a dim object. This brightness pattern, along with the galaxy’s gravitational behavior, suggests a tightly bound system with a particularly dense dark matter halo. Estimates place the invisible mass of CDG-2 at roughly 99.94 to 99.98 percent of its total mass.
In the framework of current cosmology, dark matter makes up about 27 percent of the universe’s energy density and roughly 85 percent of its matter. While we still don’t know what dark matter is made of, its presence is inferred from gravitational effects on light, ordinary matter, and the large-scale structure of the cosmos, since it neither emits nor reflects light.
Dark matter’s pervasive influence helps explain why galaxies—our Milky Way included—remain stable and move as they do. For instance, our Milky Way sits inside a dark matter halo that is thought to be predominantly dark matter.
CDG-2 represents an extreme end of the spectrum: a galaxy with almost no starlight, embedded in an almost entirely invisible halo. Such “dark galaxies” are rare, but they’re increasingly appearing in astronomical records. Beyond their rarity, these systems are valuable natural laboratories for probing the nature of dark matter and testing theories of how galaxies form.
This story originally appeared in WIRED en Español and has been translated from Spanish.
