NGC 1052-DF9: The Third Dark-Matter-Free Galaxy That Proves Dark Matter Can Be Separated from Ordinary Matter

In the vast cosmic neighborhood surrounding the giant elliptical galaxy NGC 1052, astronomers have now confirmed a third dwarf galaxy that appears to contain virtually no dark matter. The galaxy, named NGC 1052-DF9 (or simply DF9), joins two previously known oddballs—DF2 and DF4—to form a linear trail of galaxies that seem to have had their dark matter violently stripped away. This discovery, led by Michael Keim and Pieter van Dokkum at Yale University, transforms a pair of outliers into a coherent pattern, providing the strongest evidence yet that dark matter is a distinct physical substance that can be separated from ordinary matter .

The Measurement That Matters

Using the Keck Cosmic Web Imager (KCWI) at the W. M. Keck Observatory in Hawai‘i, the team measured the velocities of stars within DF9 to determine whether a dark matter halo was influencing their motion . The key metric is the stellar velocity dispersion—essentially how fast stars are moving relative to each other. If a massive halo of invisible dark matter were present, its gravity would cause the stars to move faster.

For DF9, the measured dispersion came in at 6.4 km/s (with error bars of +4.0/–4.3 km/s). This is remarkably close to the ~8.3 km/s dispersion expected from DF9's ordinary stellar mass of roughly 1.4 × 10⁸ solar masses alone . In other words, the stars are moving exactly as fast as they should if only the visible matter were pulling on them. No dark matter is required to explain the galaxy's internal dynamics.

For comparison, typical dwarf galaxies of similar stellar mass usually have velocity dispersions closer to 30 km/s or higher because of their dominant dark matter halos . DF9's low number places it firmly in the same category as DF2 (with a dispersion of roughly 3.2 km/s) and DF4 (similarly low) .

A Trail of Evidence for the Bullet Dwarf Collision

DF2 and DF4 were already famous for challenging the standard picture of galaxy formation, but their existence as a pair raised a crucial question: could they simply be two freak coincidences? The discovery of DF9—sitting right along a trail of gas and galaxies between them—makes the coincidence explanation statistically untenable .

This trail matches the predictions of the Bullet Dwarf collision scenario, a dramatic formation theory inspired by the famous Bullet Cluster. Here is how it works:

1. A high-speed crash: Two gas-rich dwarf galaxies collided head-on at extremely high velocity—roughly 358 km/s relative to each other .
2. Dark matter passes through: Because dark matter interacts only through gravity, the halos of the two galaxies passed through each other largely unimpeded.
3. Gas gets left behind: The ordinary gas and dust, which experience friction and electromagnetic forces, were stripped out and left concentrated between the separating dark matter clumps.
4. New galaxies form without scaffolding: The abandoned gas cooled and fragmented, eventually forming new ultra-diffuse galaxies—including DF2, DF4, and DF9—strung along a trail. These newborn galaxies inherited stars and gas but were born without the original dark matter scaffolding that had moved on .

The researchers estimate this collision occurred roughly eight billion years ago . The resulting galaxies share similar ages and chemical compositions, further supporting a common origin .

Why This Confirms Dark Matter Is Real

This finding strikes a blow against the leading alternative to dark matter theory: Modified Newtonian Dynamics (MOND). MOND proposes that gravity behaves differently at low accelerations, making dark matter unnecessary. If MOND were correct, every galaxy should show the same effective ratio of dynamical mass to stellar mass—the so-called "missing mass" would simply be a universal feature of gravity. You should never find a galaxy that appears to lack dark matter.

Finding not one but three galaxies in a row with normal stars and almost no evidence for dark matter breaks this symmetry. It demonstrates that the dark matter effect is not a universal law but a physical ingredient that can be physically separated from ordinary matter in violent collisions . As Pieter van Dokkum himself noted, "This is exactly what you expect if dark matter is a real substance" .

Computer simulations of high-speed dwarf galaxy collisions reinforce this. They predict exactly the kind of linear trail observed, along with a specific velocity pattern: galaxies closer to DF2 in the line should be moving faster along our line of sight than those farther away. The measured velocities of DF2, DF4, and DF9 match this prediction, adding a kinematic "smoking gun" to the morphological evidence .

The Larger Context: A Decade-Long Puzzle Solved

When van Dokkum's team first reported DF2 in 2018, the claim that a galaxy could lack dark matter was met with intense skepticism. Some researchers argued the distance to DF2 was mismeasured; others suggested tidal stripping from the nearby giant NGC 1052 could explain the missing mass .

But the subsequent discovery of DF4 in 2019, and now DF9 in 2026, has shifted the burden of proof. The Bullet Dwarf collision scenario explains the entire linear substructure naturally, while alternative explanations must account for three physically separated galaxies with similar low dispersions, similar ages, and similar chemical compositions all sitting along the same trail .

The implications extend beyond this single group. Astronomers are now searching for analogous systems elsewhere. A pair of dark-matter-deficient galaxies in the Fornax Cluster (FCC 224 and FCC 240) may represent another bullet dwarf aftermath, suggesting the phenomenon is not unique to the NGC 1052 field . Each new example reinforces the core insight: dark matter is not a modification of gravity but a real, collisionless substance that shapes the visible universe.