Astronomers Discover Another Galaxy With No Dark Matter — And It Could Rewrite Galaxy Formation Theory
For decades, dark matter has been one of the most fundamental pillars of modern cosmology. This invisible, mysterious substance — detectable only through its gravitational influence on visible matter — is thought to form the scaffolding upon which galaxies are built. Yet a growing body of evidence is challenging this assumption in a profound and unexpected way. A team of Yale University-led astronomers has now identified a third galaxy that appears to contain virtually no dark matter at all, adding significant weight to the idea that a rare and extraordinary cosmic process can produce galaxies composed almost entirely of ordinary matter.
The newly confirmed galaxy, a faint dwarf system designated DF9, was studied using the W. M. Keck Observatory atop Mauna Kea, Hawaii. The findings were published on June 16th in The Astrophysical Journal, and they are already sending ripples through the astrophysics community.
The Dark Matter Paradigm — And Why It Matters
To appreciate the significance of DF9's discovery, it helps to understand just how central dark matter is to our current picture of the universe. In the 1970s, pioneering astronomer Vera C. Rubin provided the first compelling observational evidence for dark matter by studying the rotation curves of spiral galaxies. She found that stars at the outer edges of galaxies were orbiting far too quickly to be held in place by the gravity of visible matter alone — something else, something invisible, had to be there.
Since Rubin's groundbreaking work, the indirect evidence for dark matter has accumulated dramatically. Scientists have observed gravitational lensing events, mapped vast cosmic structures, and measured the Cosmic Microwave Background (CMB) — all of which point toward the existence of an unseen form of matter that interacts gravitationally but not electromagnetically. According to the best current cosmological models, dark matter accounts for approximately 85% of all matter in the universe, or roughly 27% of its total energy content.
The prevailing theory of galaxy formation — known as the Lambda Cold Dark Matter (ΛCDM) model — holds that galaxies coalesce within enormous, invisible "halos" of dark matter. These halos act as gravitational anchors, drawing in gas and dust that eventually forms stars and the structures we recognize as galaxies. In this framework, dark matter is not merely present in galaxies — it is their very foundation.
"A line of galaxies lacking dark matter has never been seen before. The discovery provides some of the strongest evidence yet that these galaxies formed through an extreme and previously unseen process and offers a rare new window into the nature of dark matter itself." — Michael Keim, PhD Candidate, Yale University
A Trio of Anomalous Galaxies
The story of DF9 begins with two earlier, equally puzzling discoveries. In 2018, Pieter van Dokkum, the Sol Goldman Family Professor of Astronomy at Yale, led studies using data from the Hubble Space Telescope that revealed two highly unusual dwarf galaxies — DF2 and DF4 — which appeared to be nearly devoid of dark matter. These findings were initially met with skepticism; after all, a galaxy without dark matter was, by most theoretical accounts, impossible.
Now, with the confirmation of DF9 as the third such galaxy, the case has become considerably harder to dismiss. All three galaxies are part of a larger linear structure of seven galaxies located approximately 45 million light-years from Earth, in the vicinity of the massive elliptical galaxy NGC 1052. The fact that these dark-matter-deficient galaxies appear to be spatially aligned in a coherent structure is a crucial clue to their shared origin.
- DF2: The first confirmed dark-matter-deficient galaxy, identified in 2018 via Hubble Space Telescope observations.
- DF4: The second such galaxy, also identified by van Dokkum's team, further suggesting a systematic rather than random phenomenon.
- DF9: Newly confirmed as the third dark-matter-deficient galaxy, originally misidentified as a black hole before closer inspection by Keim during his doctoral research.
- All three are part of a linear chain of seven galaxies, strongly hinting at a common formation event.
- The entire structure sits roughly 45 million light-years from our own Milky Way — cosmically speaking, practically next door.
Measuring the Unmeasurable: How Scientists Weighed DF9
Determining whether a galaxy lacks dark matter is no simple task. Scientists cannot directly observe dark matter; instead, they infer its presence by measuring the gravitational forces it exerts. For galaxies, one of the most reliable methods is to study the motions of stars within a galaxy. Stars in a dark-matter-rich galaxy will orbit faster, and with more dynamical energy, than stars in a galaxy composed solely of ordinary matter.
For DF9, the team employed the Keck Cosmic Web Imager (KCWI), a state-of-the-art instrument mounted on the Keck I telescope that is specifically engineered to detect and analyze faint light sources across the universe. By measuring the velocity dispersion — essentially the spread of speeds — of stars within DF9, the team was able to calculate the galaxy's total dynamical mass.
The result was striking: DF9's total mass amounts to approximately 100 million solar masses, a figure entirely consistent with the mass of its visible stars, gas, and dust. No additional unseen mass was required to explain the stellar dynamics. If dark matter were present at the levels expected for a galaxy of its type, DF9's mass would be 100 times greater than what was measured — an enormous discrepancy that the data simply do not support.
"KCWI's exceptionally high precision enabled us to measure DF9's extraordinarily low mass with the accuracy needed to demonstrate its lack of dark matter," said lead author Michael Keim, a PhD Candidate at Yale University.
A Violent Birth: The High-Speed Collision Hypothesis
So how does a galaxy form without dark matter? The team's leading hypothesis is as dramatic as it is elegant. They propose that DF2, DF4, and DF9 all formed in the aftermath of a high-speed collision between two massive galaxies billions of years ago. During such a collision, gravitational and tidal forces can be so extreme that they physically separate ordinary matter — clouds of gas rich in star-forming material — from their dark matter halos.
In conventional galaxy mergers, dark matter and ordinary matter are stripped away together. But in a sufficiently fast and precisely oriented collision, the physics may work out differently. Ordinary matter, being more susceptible to certain hydrodynamic interactions, can be torn away and flung outward in clumps, each of which may then collapse under its own gravity to form a new, small galaxy. The dark matter, interacting only gravitationally, remains behind or disperses differently, leaving the newly formed dwarf galaxies virtually devoid of it.
The linear arrangement of the seven galaxies in this structure lends strong geometric support to this scenario. A single, directional event — like a high-velocity galactic impact — would naturally produce debris strung out along a line, much like pearls on a string. Computer simulations of such an event have proven remarkably consistent with what astronomers observe.
Pieter van Dokkum added an important theoretical dimension to these findings: "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."
This last point deserves emphasis. Some physicists have proposed alternatives to dark matter — theories such as Modified Newtonian Dynamics (MOND) — which attempt to explain galactic rotation curves and other phenomena by modifying the laws of gravity rather than invoking an invisible substance. However, these alternative theories generally predict that all galaxies should exhibit similar gravitational signatures. The existence of galaxies that genuinely lack any dark matter-like effect is deeply problematic for such theories, and strongly supports the interpretation that dark matter is a real, physical form of matter.
Implications for Cosmology and Future Research
These findings carry far-reaching implications for our understanding of galaxy formation and evolution. The dominant ΛCDM model has been extraordinarily successful at explaining the large-scale structure of the universe, but anomalies like DF2, DF4, and DF9 reveal that it may be incomplete when it comes to describing the full diversity of galaxy types that can exist.
The discovery also opens a powerful new observational window. If galaxies can form without dark matter through high-speed collisions, then identifying and studying more such systems could allow astronomers to effectively "subtract" the influence of dark matter and study ordinary matter dynamics in isolation — a rare and precious experimental control that the universe rarely offers.
Looking ahead, the team is planning an ambitious follow-up observational campaign. They are searching for residual gas left behind from the theorized galactic collision — a kind of cosmic fingerprint that would serve as direct evidence of the event. This work will be conducted using multiple telescopes, including the newly developed Mothra telescope, co-founded by van Dokkum and astronomer Roberto Abraham of the University of Toronto. Future next-generation observatories, including the Vera C. Rubin Observatory (fittingly named after the pioneer of dark matter research) and the James Webb Space Telescope, may also play a role in uncovering more galaxies of this type and testing the collision hypothesis with unprecedented precision.
Key Takeaways
- A team led by Yale University astronomers has confirmed DF9 as the third known galaxy to contain virtually no dark matter.
- DF9's total mass is approximately 100 million solar masses — consistent solely with visible matter, and 100 times less than expected if dark matter were present.
- DF9 is part of a linear chain of seven galaxies located 45 million light-years away, alongside the previously identified dark-matter-deficient galaxies DF2 and DF4.
- The team proposes that all three galaxies formed from a high-speed galactic collision that separated ordinary matter from dark matter halos billions of years ago.
- The findings challenge alternative gravity theories like MOND and support the interpretation that dark matter is a real physical substance.
- The study was published in The Astrophysical Journal and used data from the W. M. Keck Observatory and the Hubble Space Telescope.
Ultimately, the discovery of DF9 is a reminder of the universe's remarkable capacity to surprise us. In a cosmos thought to be dominated by invisible dark matter, these three small, faint galaxies — drifting quietly through space, stripped of the very substance that supposedly makes galaxies possible — are asking us to think more carefully, and more creatively, about the processes that shape the cosmos we call home. For more information, visit the Yale News press release and the full study in The Astrophysical Journal.
