At the center of many galaxies lurks an invisible cloud — a halo of dark matter. If this matter is cold and light enough, it behaves like a superfluid: it flows without friction, and any rotation in it twists into tiny quantum vortices — threads the size of an atomic nucleus. In a halo thousands of light-years across, such vortices could be countless.
When vortices collide, they reconnect, like two wisps of smoke that briefly merge and then separate differently. Some energy goes into sound waves (dark phonons) and small ripples. The authors calculated how much this process can heat and stir the matter in the halo's core. They took simulation data and applied it to eight dwarf satellite galaxies of the Milky Way, such as the galaxy Draco or Sculptor.
The result was modest: even with the most generous estimates, vortices deliver only a small fraction of a percent of the core's energy. The speed of matter changes by no more than 2.3%. If there are no vortices in the core, there is no heating at all. So vortex dark matter remains almost as cold and calm as previously imagined.
A quantum vortex in superfluid dark matter is essentially a tiny tornado where matter doesn't rotate as a whole but moves only around an ultra-thin core.