Keck Finds DF9, a Third Galaxy That Appears to Lack Dark Matter

DF9 has become the third known galaxy that appears to contain little or no dark matter, joining DF2 and DF4. The study, led by Michael Keim of Yale University, was published in The Astrophysical Journal, and the W. M. Keck Observatory announced it on June 16, 2026. All three galaxies lie in a narrow line of faint galaxies in the NGC 1052 region.

The team used the KCWI instrument at Keck to measure how fast DF9’s stars move. Faster motions mean more mass is holding the galaxy together. The result is a total mass of only about 100 million Suns, consistent with its visible stars and gas alone. A galaxy like DF9 with a typical amount of dark matter would be about 100 times more massive.

How could a galaxy lose its dark matter? The team proposes that the galaxies in the line formed together in a high-speed collision that separated gas from its dark matter. Dark matter passes through itself, while gas collides and slows down, so the two can be pulled apart. That is the same physics seen on a much larger scale in the Bullet Cluster, studied in 2006.

Why it matters: galaxies without dark matter are hard to explain with theories that replace dark matter with modified gravity, because modified gravity should affect every galaxy alike. Co-author Pieter van Dokkum says the result supports dark matter as a physical substance at dwarf-galaxy scales. These galaxies are natural experiments, but three is still a small sample and the collision scenario remains a proposal. More in our dark matter guide.

What to watch next: stellar motions for other galaxies in the same line, to test the collision scenario.