Dark matter acts as the invisible glue holding the largest cosmic structures together. It pulls ordinary, visible matter inward, enabling stars to form and preventing the sprawling arms of galaxies from flying apart. Shyam Balaji, a theoretical physicist at King’s College London, notes that almost all galaxies are dominated by dark matter.
The Discovery of Anomalous Galaxies
However, three diffuse galaxies known as DF2, DF4, and DF9 have left astronomers stunned. By utilizing a suite of telescopes to estimate their masses through stellar motions, researchers discovered that all three possess very little dark matter. Instead, a tiny fraction of ordinary matter alone forged their sparse population of stars. How could these rule-breaking galaxies have formed?
A Cosmic Collision Theory
A paper published in June 2026 in The Astrophysical Journal offers a potential solution. Long ago, two regular galaxies collided. Their ordinary matter, consisting of gas and dust, swirled together and formed clumps. Meanwhile, the dark matter passed straight through without obstruction, acting much like a specter moving through a wall. This cosmic pileup left behind three new galaxies completely devoid of dark matter, struggling to generate a new batch of stars.
Proving the Physical Existence of Dark Matter
This explanation convinces many experts and crucially sheds light on the broader mystery of dark matter. Some scientists suggest that dark matter, which remains undetected directly, might not exist at all. They propose instead that tweaking our current understanding of gravitational force could account for various features across the universe.
Challenging Modified Gravity Theories
These bizarre galaxies pour cold water on that hypothesis. Keim, an author of the new study, explains that this demonstrates how dark matter is not merely a modification of gravity. Separating ordinary matter from dark matter should result in dark-matter-lacking galaxies struggling to form stars, which matches the behavior observed in DF2, DF4, and DF9.
Dark Matter as a Physical Substance
This evidence supports the long-held suspicion that dark matter is a physical substance. Balaji points out the irony that the absence of dark matter provides indirect evidence for it. Daniel Whiteson, a particle physicist at the University of California, Irvine, adds that while the correct dark matter candidate remains unknown and undetected, multiple independent lines of evidence indicate that dark matter is indeed physical matter.
The Vital Role of Dark Matter
Accounting for 27 percent of the universe and 85 percent of all matter, dark matter interacts with gravity. Without its influence, stars at the edges of rapidly spinning galaxies would escape into open space, and ordinary matter alone would fail to clump tightly enough to initiate star formation. Dark matter also warps space, bending light around massive objects through gravitational lensing.
The Impossibility of Dark-Matter-Free Galaxies
Chamkaur Ghag, a physicist at University College London, emphasizes that galaxies could not exist without dark matter, noting that current understanding rules out any natural evolutionary path for galaxies without it. When DF2 was discovered in 2018, astronomers struggled to classify it properly.
Examining DF2, DF4, and DF9
Roughly the size of the Milky Way, DF2 contains 200 times fewer stars, making it significantly dimmer. Suspicious astronomers used the Keck Observatory in Hawaii to track stellar motions and estimate the galaxy's total mass. They discovered an unexpectedly lightweight structure containing at most 400 times less dark matter than anticipated, designating DF2 as the first known dark-matter-deficient galaxy.
Expanding the Phantom Galaxy Club
In 2019, scientists observed DF4, another phantom galaxy lacking dark matter. By 2026, a third member, DF9, joined the group. DF9 possesses the mass of roughly 100 million suns, consistent with containing only ordinary matter rather than being a hundred times more massive as it would be if packed with dark matter.
Tracing the Origin Story
Astrophysicists noted that all three gossamer galaxies aligned as neighbors, sparking investigations into their origin story. Because dark matter is known to interact exclusively via gravity without compelling evidence of engaging with other fundamental forces like electromagnetism, a head-on collision between typical galaxies would allow dark matter to pass with minimal friction or drag while ordinary matter becomes severely tangled.
The Jousting Analogy
Aaron Romanowsky, an astronomer at San Jose State University, compares this to two knights jousting, where the knights and lances collide and fall into a heap while their horses continue running at full speed. This resembles the Bullet Cluster, a massive collection of galaxies located 3.8 billion light-years away that collided long ago. Recent observations by the James Webb Space Telescope reveal a messy mass distribution that makes sense only if dark matter and ordinary matter separated during the impact.
Alternative Explanations for the Separation
Although this segregation appears clearly in DF2, DF4, and DF9, scientific consensus remains divided on the exact mechanism of separation. Kinwah Wu, a theoretical astrophysicist at University College London, points out that the triplet lacks the fluid, wispy features expected following a galactic collision. Instead, he suggests that DF2, DF4, and DF9 passed through a massive galaxy cluster, which stripped away their dark matter during the voyage.
Skepticism Among Researchers
Stacy McGaugh, an astronomer at Case Western Reserve University and an advocate for modified-gravity theories, remains skeptical. He questions the formation mechanism, describing an ancient collision as a plausible yet exceptional origin story. He argues that even if that scenario occurred, confirming the exact origin is necessary before drawing conclusions about the fundamental nature of dark matter.
The Scientific Consensus
Most experts maintain that explaining the existence of DF2, DF4, and DF9 without treating dark matter as a physical entity is extremely difficult. Ghag reiterates that evolving such objects independently without dark matter defies current physical models.
A Major Blow to MOND
This galactic trio poses significant problems for Modified Newtonian Dynamics, or MOND, an alternative to dark matter proposed in 1983 by physicist Mordehai Milgrom. Whiteson explains that MOND suggests visible matter operates under altered gravitational laws when experiencing very weak gravitational pulls across vast distances, potentially holding spinning galaxies together without dark matter.
The Impact of LIGO Discoveries
Cosmologists favored dark matter because MOND consistently failed to explain all phenomena. Ghag notes that MOND suffered a major setback in 2015 when the US-based Laser Interferometer Gravitational-Wave Observatory detected gravitational waves for the first time. The discovery proved that these waves travel at the speed of light across vast distances in all directions, confirming consistent and unmodified gravitational behavior throughout the cosmos.
Why Removal Beats Modification
Balaji argues that removing cold dark matter through a collision is far easier than altering a fundamental law of nature, making dark matter a more elegant explanation for DF2, DF4, and DF9.
Continuing Scientific Debate
While Whiteson views alternative theories and open debate as healthy for science, he stresses the importance of evaluating whether theories align with empirical data, pointing out that MOND falls short. While the ultimate identity of dark matter remains elusive, these unusual galaxies present another major challenge to its struggling theoretical competitor.
