A galaxy collision may explain why DF2, DF4 and DF9 hold so little dark matter
Astronomers writing in The Astrophysical Journal in June 2026 propose that a collision of two galaxies left DF2, DF4 and DF9 with little dark matter. An author says the pattern counts against gravity-only explanations, though the candidate particles stay open.
The Scientist · Science desk
Key figures: DF2 seemed to contain at most 400 times less dark matter than would be expected. DF2 has 200 times fewer stars than the Milky Way, though it is roughly the same size. DF2, DF4 and DF9 are about 65 million light-years from Earth.
| Figure | Value | Note | Claim |
|---|---|---|---|
| Less dark matter in DF2 than expected, at most | 400 times less | 8 | |
| DF2 stars vs Milky Way | 200 times fewer | 7 | |
| Distance from Earth, about | 65,000,000 light-years | 1 |
What happened
- The three galaxies sit about 65 million light-years from Earth and look pallid, dimly lit by a smattering of stars.
- DF2, found in 2018, is about the size of the Milky Way but has 200 times fewer stars, and Keck observations of those stars gave its mass estimate.
- Masses came from the motions of each galaxy's stars, measured with a suite of telescopes and used as a proxy for total mass.
- In the proposed scenario, gas and dust from two colliding galaxies clumped together while their dark matter passed through and carried on.
Why it matters
- What it limits constraint Modified-gravity ideas now have to explain why galaxies that lost their dark matter also struggle to form stars, the pattern the collision scenario predicts.
- What it limits constraint The result narrows the question only between a physical substance and a gravity tweak. Neutrinos, WIMPs and primordial black holes are all still candidates.
- What doesn't add up contradiction The expectation that galaxies need dark matter meets three galaxies with little of it. The scenario reconciles them by treating the trio as products of a split, an inference from present-day star motions.
The scenario's main prediction concerns star formation. Separate ordinary matter from dark matter, and the galaxies left without dark matter should struggle to form stars; that is what DF2, DF4 and DF9 show [4]. Michael Keim, an astrophysicist at Yale University and an author of the paper, drew the conclusion directly. "What this is showing is that dark matter is ... not just a modification of gravity," he said [10].
That matters because some scientists have proposed that dark matter may not exist and that a tweak to gravity could explain the features it was invented for [11]. Shyam Balaji, a theoretical physicist at King's College London, put the logic this way: "The absence of dark matter is, in a way, some evidence of dark matter, which is ironic" [12]. Quanta reports that the result suggests dark matter is a physical thing, as many have suspected for some time [14]. We think that is a fair reading of the star-formation pattern, on one condition. The masses come from stellar motions, which tell you what a galaxy contains now [2], and the collision history is inferred from them.
The result supports dark matter as a substance. It does not pick out a particle. Daniel Whiteson, a particle physicist at the University of California, Irvine, said: "We don't know which dark matter candidate is the right one" [5]. Quanta lists a stealthy type of neutrino, WIMPs and atom-sized primordial black holes among the proposals [6]. Nothing in the account ties the collision scenario to any of them.
The standard expectation is stated flatly by Chamkaur Ghag, a physicist at University College London: "You wouldn't get galaxies without dark matter" [13]. The collision picture keeps that rule intact by treating the three galaxies as products of a split between the two kinds of matter [3].
The sample is small: three galaxies and one proposed history. For DF2, the first dark matter-deficient galaxy found, Quanta reports a mass that seemed to hold at most 400 times less dark matter than expected [8]. Quanta calls the scenario "a convincing explanation for many experts" [9]. The material reviewed here does not include the paper's own mass estimates for DF4 and DF9 or any rival explanation for the trio.
What to watch
- Independent mass estimates for DF2, DF4 and DF9, since the whole argument rests on stellar motions as a proxy for total mass.
- Whether other dark matter-deficient galaxies turn up and show the same weak star formation the collision scenario predicts.
- Any modified-gravity account that explains both the missing dark matter and the poor star formation in these three galaxies.
Clarity's read
What the record supports and how the coverage leans. The claims behind it follow.
Reality
- Evidence55
- Adoption
- Not scored
- Hype gap+10
- Incentives
- Not scored
- Confidence55
Claim ledger
Ranked by verification strength, evidence, and original report placement.
- [1]
About 65 million light-years from Earth, three galaxies, DF2, DF4 and DF9, look pallid and are dimly illuminated by a smattering of stars.
- [2]
Researchers used a suite of telescopes to estimate the masses of DF2, DF4 and DF9 using the motions of their stars as a proxy, and found that all three have little dark matter.
- [3]
A paper published in June 2026 in The Astrophysical Journal proposes that two regular galaxies collided; their gas and dust clumped up while the dark matter passed through and continued on, leaving three new galaxies each lacking dark matter.
- [4]
If you segregate ordinary matter from dark matter, you would expect the dark matter-lacking galaxies to struggle to form stars, and that is what DF2, DF4 and DF9 show.
- [5]
Daniel Whiteson, a particle physicist at the University of California, Irvine, said: "We don't know which dark matter candidate is the right one."
- [6]
Proposed dark matter candidates include a stealthy type of neutrino, weakly interacting massive particles (WIMPs), and primordial black holes as small as individual atoms and as old as the universe.
- [7]
DF2, discovered in 2018, is roughly the same size as the Milky Way but has 200 times fewer stars; scientists used the Keck Observatory in Hawai'i to track the motions of its stars to estimate its total mass.
- [8]
DF2 seemed to contain, at most, 400 times less dark matter than would be expected, making it the first dark matter-deficient galaxy ever spotted.
- [9]
Quanta Magazine writes that the collision scenario is "a convincing explanation for many experts."
- [10]
Michael Keim, an astrophysicist at Yale University and an author of the new study, said: "What this is showing is that dark matter is ... not just a modification of gravity."
- [11]
Some scientists have suggested that dark matter, which has yet to be directly detected, may not exist, and that a tweak to our understanding of the gravitational force might explain various features of the universe.
- [12]
Shyam Balaji, a theoretical physicist at King's College London, said: "The absence of dark matter is, in a way, some evidence of dark matter, which is ironic."
- [13]
Chamkaur Ghag, a physicist at University College London, said: "You wouldn't get galaxies without dark matter."
- [14]
Quanta Magazine writes that the result suggests, as many have suspected for some time, that dark matter is a physical thing.
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Sources
1 independent publisher whose own reporting we read for this story.
- quantamagazine.orgTriplet of ‘Ghostly’ Galaxies Hints at What Dark Matter Is — and Isn’t
1 article · October 14, 2026
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