This Galaxy Is 99% Dark Matter—and Basically Invisible

Dark matter doesn’t emit, absorb, or reflect light. It’s invisible but supposedly makes up 85% of the universe’s mass. Because it’s so abundant, astronomers believe it should explain many unsolved mysteries surrounding the cosmos, including an extraordinary dim galaxy “hiding” in plain sight.

Last summer, astronomers vetted candidates for so-called dark galaxies, or a rare class of galaxies with remarkably low surface brightness. The paper on the findings, published in The Astrophysical Journal Letters, surmised that so-called “dark galaxies” should have “some of the most extreme properties among all known galaxies.”

And today, NASA shared high-resolution images of one such galaxy, CDG-2. If the astronomers’ hypothesis is correct, CDG-2 would be among the most heavily dark-matter-filled galaxies ever identified.

“Seeing” the invisible

Again, dark matter doesn’t interact with light. However, astronomers infer its presence from dark matter’s gravitational influence on surrounding objects. Often, this refers to unseen mass that seems to be driving galaxies, planets, stars, and supernovas—you name it—to appear a certain way.

Simply put, if dark matter didn’t exist, many of the models scientists use to understand the cosmos would break down. Although scientists have yet to directly detect dark matter, dissenters have also yet to find an alternative that explains things better. So the consensus, at least for now, is that dark matter exists.

Hints of an invisible galaxy

CDG-2 lies within the Perseus galaxy cluster, a section of the universe filled with globular clusters, or tightly bound units filled with millions of stars. Accordingly, this neighborhood is known for its particularly rich population of densely packed stars.

Then, Hubble noticed a close collection of four globular clusters among the bunch. When Hubble, ESA’s Euclid, and the Subaru Telescope in Hawaii collectively took a closer look, astronomers noticed a faint glow circling the area—leading them to wonder if the whole thing was actually just one galaxy.

The researchers behind the study conducted a statistical analysis of the galaxy, finding that the four clusters likely are part of a single galaxy—and a dark one at that.

“This is the first galaxy detected solely through its globular cluster population,” David Li, the study’s lead author and an astronomer at the University of Toronto, said in a NASA release.

A crowded neighborhood

According to the researchers, CDG-2 has a luminosity equivalent to roughly 6 million Sun-like stars, with the globular clusters contributing around 16% of that total. In terms of mass, however, 99% of the galaxy’s mass appears to be dark matter.

The “normal” matter in CDG-2—primarily hydrogen gas for star formation—was likely stripped away by the dense population of galaxies inside the Perseus cluster, the scientists said. But even if CDG-2 isn’t necessarily a dark galaxy, it’s still an odd one, the researchers said, and would be an ideal candidate to study and test theories on clustered star formation and its potential implications, according to the paper.

Either way, it looks like astronomers have no shortage of things they’re counting on dark matter to prove.