The above image might not look unusual, but it poses an interesting mystery for astronomers. Captured by the Hubble Space Telescope, it is an image of an ultra-diffuse galaxy (UDG) known as FCC 224. Unlike regular dwarf galaxies such as the Magellanic Clouds, these galaxies have an unusually low surface brightness for a galaxy of their size. This usually means they contain lots of dark matter. The gravity of the dark matter holds the galaxy together rather than the stars and cosmic dust.
The standard cosmological model predicts UDGs. Since dark matter is more common than regular matter, it’s the clumping of dark matter that drives galaxy formation. Sometimes a dark matter clump can form without a lot of stellar material around, and you get a galaxy without lots of stars. But in recent years, astronomers have found that some diffuse galaxies aren’t dominated by dark matter. The evidence pointing to a lack of dark matter wasn’t definitive, but it was strong enough for astronomers to wonder about how such a galaxy could form. One idea is that they are a result of a recent collision that stripped the diffuse galaxy of its dark matter. Other astronomers argue that such a galaxy would support alternative models, such as modified gravity.
This new galaxy has two things going for it. The first is that FCC 224 is not part of a galactic cluster known as the NGC 1052 group. This means we can just argue that these odd UDGs are the result of some unique property of a particular cluster. In one study, the authors show that there are shared characteristics among these galaxies, such as having old, metal-poor stars, low levels of dust, and a slow rate of rotation. This suggests that dark matter free ultra-diffuse galaxies can be categorized as a specific type of galaxy.
A second study looks at globular clusters with FCC 224. They are unusually small and tightly packed, which would throw a wrench in the collision model. If a collision was strong enough to strip the galaxy of dark matter, it should also have been strong enough to disrupt the globular clusters.
Overall, these studies confirm the existence of ultra-diffuse galaxies without dark matter, and their existence suggests we may have to revise the standard model.
Reference: Buzzo, Maria Luisa, et al. “A new class of dark matter-free dwarf galaxies? I. Clues from FCC 224, NGC 1052-DF2 and NGC 1052-DF4.” arXiv preprint arXiv:2502.05405 (2025).
Reference: Tang, Yimeng, et al. “An Unexplained Origin for the Unusual Globular Cluster System in the Ultra-diffuse Galaxy FCC 224.” arXiv preprint arXiv:2501.10665 (2025).