As if the universe weren’t strange enough, scientists have recently discovered that entire galaxy clusters—the largest known structures in the universe, consisting of thousands of galaxies—are moving toward the same area. And we have no idea what mysterious phenomenon is drawing them along. Whatever it is, it’s huge. So far, cosmologists’ best guess is that it’s the gravitational pull from something beyond the visible universe. NASA scientist Alexander Kashlinsky and a team of researchers discovered the mystery motion, dubbed “dark flow,” last year.