Inside a dwarf galaxy lives an old ball of stars, a gathering of hundreds of thousands of suns. By all calculations, it should have sunk to the center long ago, like a stone in thick syrup. But the ball holds its ground, and scientists can't figure out why.

Around the galaxy sits invisible dark matter. It doesn't shine, but it pulls on stars. In the standard model, dark matter clumps toward the center, and any massive object flying through it gets slowed down, like a boat dragging a wave behind it. Over ten billion years, that kind of friction should have pulled every globular cluster into the center. But telescopes show them alive and scattered across the whole galaxy.

One explanation: dark matter is made of ultralight particles that behave like waves. These waves blur out the galaxy's dense core, and friction against dark matter becomes weaker. Scientists tested this idea on data from the Euclid telescope for hundreds of dwarf galaxies. It turned out that if the dark matter particle is lighter than a certain limit, then globular clusters don't sink to the center. That limit agrees with some observations but contradicts others. For now the puzzle remains, but new data will help solve it.