Nathaniel Starkman

Stars on the outskirts of galaxies rotate faster than predicted by the laws of physics if only visible matter is considered. This paradox was uncovered in the 20th century by Vera Rubin and Fritz Zwicky.

A stellar river winds around the center of a distant galaxy like a glowing ribbon on an invisible frame. That frame is the dark halo: a vast cloud of invisible dark matter whose gravity holds stars and gas in place. If the halo is squashed like a pumpkin or stretched like a melon, the stellar stream will bend differently.