Michael Keim was poring over data from a misidentified black hole when he realized he wasn’t looking at a singularity at all—just a faint, ghostly galaxy 45 million light-years away, glowing with the light of a hundred million suns and missing something fundamental: dark matter. That galaxy, NGC 1052-DF9, is now the third known galaxy without dark matter—and it’s not alone. It lies in a straight line with nine other galaxies, including two previously discovered dark matter-free dwarfs, DF2 and DF4, forming a cosmic alignment never seen before. This narrow chain of faint, diffuse galaxies stretches across space like a thread pulled through the fabric of the universe, offering astronomers an unprecedented look at how galaxies can form outside the gravitational scaffolding long thought to be essential.
For decades, the prevailing theory has held that galaxies form within massive halos of dark matter—its invisible gravity pulling gas together until stars ignite. But DF9, DF2, and DF4 defy that model. Using the W.M. Keck Observatory’s Cosmic Web Imager, Keim and his advisor, Yale astronomer Pieter van Dokkum, measured the motion of stars within DF9 and found its total mass to be just 100 million times that of the Sun—entirely accounted for by visible matter. If dark matter were present in the expected amount, the galaxy’s mass would exceed 10 billion suns. The absence is stark, and telling.
The discovery, published in The Astrophysical Journal, strengthens the case that these galaxies were born from a violent, high-speed collision—one that ripped gas away from dark matter, sending it trailing through space like debris from a cosmic crash. Within that stream of ordinary matter, new galaxies formed, starved of the dark substance thought to anchor all others. "A line of galaxies lacking dark matter has never been seen before," said Keim, lead author of the study. "This system shows that stars and galaxies can form outside of dark matter 'halos' in extreme events and indicates that dark matter is a physical substance that can act independently of normal matter or gas."
That conclusion is revolutionary. It challenges alternative theories that attribute dark matter’s effects to modifications of gravity, instead supporting the idea that dark matter is a real, separable material. The finding reaffirms van Dokkum’s earlier work on DF2 and DF4, which first sparked debate in the astrophysics community. Now, with DF9 as a crucial third data point, the evidence is harder to dismiss.
The team is now turning newer tools, like the Mothra telescope—a project co-founded by van Dokkum and University of Toronto astronomer Roberto Abraham—toward the region, searching for residual gas left behind by the ancient collision. If found, it could trace the full path of this galactic scar and reveal how often such events occur. For now, the linear trio stands as a quiet but powerful testament: the universe still holds mysteries, and sometimes, the absence of something makes all the difference.