An unusually dense galaxy could be the first clear evidence for the existence of an unconventional form of “sticky” dark matter, altering our understanding of this mysterious cosmic substance.

In the standard picture of cosmology, so-called cold dark matter only interacts with the rest of the universe through gravity, which causes it to bunch together in invisible, puffy clouds around galaxies. We can map these clouds indirectly by measuring the gravitational pull they exert, which bends…