Scientists typically believe that dark matter, for all of its mystery, behaves in a simple way: if one clump encounters another, the two interact solely through gravity. However, researchers using both Hubble and the Very Large Telescope have published findings which suggest that there's more involved. They've noticed dark matter (the blue lines in this picture) lagging behind a galaxy due to friction, hinting that there are factors beyond gravity at work. It's not certain whether the source of this friction is a familiar phenomenon or something entirely undiscovered, but it's definitely not the usual culprit.