About 65 million light-years away, three dim galaxies known as DF2, DF4, and DF9 have left astronomers puzzled. Most galaxies are dominated by dark matter, the invisible substance that helps pull ordinary matter together and keeps galaxies intact. But measurements of the motions of stars in these three galaxies suggest they are made almost entirely of ordinary matter, with little or no dark matter at all.
A study published in June 2026 in The Astrophysical Journal proposes an origin for these odd objects. Two ordinary galaxies may have collided head-on. Their gas and dust clumped together, while their dark matter passed through the collision almost without interacting, leaving behind three new galaxies that are bereft of dark matter. Because dark matter is thought to interact only through gravity, it would experience little friction during such a crash, unlike ordinary matter.
The existence of these galaxies is significant because some scientists have argued that dark matter may not exist, and that tweaks to our understanding of gravity could explain cosmic observations instead. The new findings pour cold water on that idea. If dark matter were merely a modification of gravity, separating ordinary matter from dark matter would be hard to explain. As one researcher put it, the absence of dark matter in these galaxies is, ironically, evidence that dark matter is real.