An old stellar ball that has gathered hundreds of thousands of suns has been circling the edge of a dwarf galaxy for billions of years. By all calculations, it should have long ago plunged into the center like a heavy stone into jelly. But the ball holds its ground, and this fact has been nagging astrophysicists for several decades.

In the standard picture of cold dark matter, galaxies are surrounded by an invisible halo whose density rises sharply toward the center. Any massive object moving through such a halo experiences drag: the gravity behind it is stronger than in front, and it slows down like a boat pulling a wave along. This effect is called dynamical friction. For globular clusters in dwarf galaxies, it should be so strong that within ten billion years they would all end up in the center. But telescopes see the opposite: the clusters are alive and spread throughout the entire volume.

The solution may lie in the idea that dark matter is not cold but fuzzy. In this model, the dark matter particle is so light that its wave properties manifest on scales of thousands of light-years. Instead of a sharp density peak at the galaxy's center, a smooth core forms, resembling a blurred patch of fog. Dynamical friction in such fog is weaker, and the clusters stop sinking.

The authors of the new work used early observations from the Euclid space telescope for two galaxy clusters: Fornax and Perseus. They selected thousands of globular cluster candidates in hundreds of dwarf galaxies and traced their orbits over ten billion years. By varying the mass of the fuzzy dark matter particle, they found the threshold at which clusters stop falling toward the center. For the Fornax cluster, the upper limit came out to 1.46×10⁻²² eV, and for Perseus, 1.09×10⁻²² eV. These numbers agree with estimates from stellar dynamics in dwarf galaxies but contradict data on stellar streams and the Lyman-alpha forest.

For now it is unclear who is right. Perhaps the fuzzy dark matter model needs refinement, or perhaps the timing problem is solved differently, for example, clusters form farther from the center than previously thought. Future Euclid data releases will provide an order of magnitude more objects and will help narrow the allowed mass range. Meanwhile, the old stellar ball continues its dance on the outskirts, refusing to sink into the fog.