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A Yale astrophysics Ph.D. student, Michael Keim, has identified a galaxy called NGC 1052-DF9 (DF9) that appears to contain little or no dark matter. Its importance comes from its position: it sits in a narrow line with two previously reported galaxies, DF2 and DF4, which also appear to lack most of their dark matter. Together, the three may point to a violent collision that pulled ordinary gas away from dark matter. That explanation is a hypothesis supported by the new evidence, not a settled answer.
What Keim found
Keim identified DF9 during his doctoral work. According to Yale News, DF9 had previously been misidentified as a supermassive black hole. Keim then proposed a detailed analysis using the Cosmic Web Imager (KCWI), an instrument on the W. M. Keck Observatory’s telescope in Hawaii. That analysis is what turned a misidentified object into a galaxy with an unusually low mass.
DF2, DF4 and DF9: the three galaxies in the headline
The headline’s “three galaxies” are three faint, diffuse galaxies that form a narrow line. The Keck Observatory release describes DF2, DF4 and DF9 as extraordinary exceptions to the usual picture in which dark matter dominates galaxies. The broader structure contains other faint galaxies, but only these three are described as lacking dark matter.
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- DF2 and DF4 were reported earlier as galaxies with little or no dark matter.
- DF9 is the third example, and Yale places it about 67 million light-years from Earth.
How astronomers tell whether a galaxy has dark matter
Dark matter cannot be seen directly. Astronomers infer it from gravity. Stars orbit within a galaxy at speeds set by the total mass pulling on them, visible matter and dark matter alike. If stars move faster than their visible light would account for, there is more unseen mass; if they move slowly, the galaxy has little hidden mass.
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For DF9, the team used KCWI to analyze light at different wavelengths, reading the shifts that reveal how stars are moving. From those motions they inferred the galaxy’s total mass. Keck reports that the result, about 100 million solar masses, is consistent with the mass of DF9’s visible matter alone. The comparison below shows why that matters.
| Quantity | Reported value | Source and status |
|---|---|---|
| DF9 total mass inferred from stellar motions | About 100 million solar masses | W. M. Keck Observatory, June 2026; an inference from measurements, consistent with visible matter |
| Expected mass if DF9 had a typical amount of dark matter | About 100 times the inferred mass | W. M. Keck Observatory, June 2026; a comparison, not a measurement |
| Same comparison expressed as a total | More than 10 billion solar masses | Yale News, June 2026; a comparison, not a measurement |
| Distance from Earth | About 67 million light-years | Yale News, June 2026 |
The logic is comparative. The team does not have to see dark matter to argue that it is missing. It only has to show that the mass implied by stellar motion is far closer to the visible matter than the usual galaxy budget would allow.
The collision hypothesis
The mystery is why three neighboring galaxies would lack dark matter. The researchers propose a sequence of events, which is a scenario rather than an observed fact:
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- Two large structures collided at high speed.
- The collision separated the ordinary gas from the dark matter surrounding it.
- The gas then condensed into galaxies that formed along a line, leaving the dark matter behind.
The new evidence is described as strengthening this scenario. It is not proof. A key test is still ahead: the team plans to search for gas that the collision may have left behind, and to constrain how much gas is in the three galaxies. If that gas is found where the scenario predicts, confidence will rise; if it is not, the collision idea will need revision.
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What the finding does and does not settle
The finding has a clear scientific claim and a larger, more debated implication. Keim described the observation this way: “A line of galaxies lacking dark matter has never been seen before.” That novelty is the core of the result.
The larger implication concerns the physics of dark matter. Pieter van Dokkum, a Yale astronomer and co-author of the study, argued that the finding “provides compelling evidence that dark matter behaves as a physical substance rather than the effect of an alternative theory of gravity, particularly at the dwarf-galaxy scale where those theories are most heavily debated.” That is van Dokkum’s interpretation, and alternative gravity theories remain part of the debate.
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Several limits apply:
- The evidence rests on one group of galaxies. It does not, by itself, settle the broader debate.
- “Without dark matter” is a qualified description. Reports say the galaxies appear to contain little or no dark matter, based on inferred mass.
- The nature of dark matter is not solved. A physical substance that can be separated from ordinary matter is one possibility this system supports, not a confirmed identity for it.
The institutional releases state that the study was published in The Astrophysical Journal on June 16, 2026.
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