Triplet of ‘Ghostly’ Galaxies Hints at What Dark Matter Is — and Isn’t
A collision may have left these galaxies without dark matter. That spells trouble for alternative gravity theories.
In 2018, 2019, and 2026, astronomers discovered three faint galaxies named DF2, DF4, and DF9, located about 65 million light-years from Earth. These galaxies appear dim and diffuse, earning them the nickname 'ghostly galaxies.' Unlike most galaxies, which are held together by dark matter, these three contain almost no dark matter. For example, DF2 is roughly the size of the Milky Way but has 200 times fewer stars and at most 400 times less dark matter than expected. This discovery challenged long-held beliefs about how galaxies form and what keeps them stable.
Dark matter is an invisible substance that makes up 27% of the universe and 85% of all matter. It interacts only with gravity, not with light or other forces, making it undetectable by telescopes. Dark matter acts as a cosmic glue, holding galaxies together and preventing stars on their edges from flying into space. Without dark matter, galaxies like the Milky Way would not form or maintain their structure. The discovery of DF2, DF4, and DF9, which lack dark matter, puzzled scientists because these galaxies should not exist without it.
A 2026 study suggests that DF2, DF4, and DF9 may have formed after a violent collision between two larger galaxies. During such a collision, dark matter—being invisible and interacting only with gravity—would pass through the galaxies like a ghost, while ordinary matter (gas and dust) would collide and clump together. This process could have left behind three new galaxies composed almost entirely of ordinary matter, stripped of their dark matter. The study was led by Michael Keim from Yale University and published in The Astrophysical Journal in June 2026.
Some scientists have proposed that dark matter might not exist and that instead, gravity behaves differently on large scales, a theory known as modified Newtonian dynamics (MOND). MOND suggests that gravity weakens less than expected over vast distances, which could explain why galaxies like the Milky Way stay intact without extra dark matter. However, the discovery of dark matter-deficient galaxies like DF2, DF4, and DF9 challenges this idea. Experts argue that it is easier to explain these galaxies by removing dark matter through a collision than by altering the laws of gravity.
The existence of DF2, DF4, and DF9 provides indirect evidence that dark matter is a physical substance rather than a modification of gravity. Daniel Whiteson, a particle physicist at the University of California, Irvine, stated that the absence of dark matter in these galaxies is itself evidence for its existence. Similarly, Chamkaur Ghag from University College London noted that galaxies cannot form or survive without dark matter. While the origin story of these galaxies is still debated, most experts agree that their existence supports the idea that dark matter is a real, tangible component of the universe.
Despite decades of research, the exact nature of dark matter remains unknown. Candidates include weakly interacting massive particles (WIMPs), stealthy neutrinos, or even primordial black holes. None of these have been directly detected. The discovery of DF2, DF4, and DF9 does not solve the mystery but adds another piece to the puzzle. It reinforces the idea that dark matter is a physical substance, though its exact properties and interactions continue to elude scientists. The search for dark matter remains one of the biggest challenges in modern astrophysics.
