Nearly a century ago, a Swiss physicist at the California Institute of Technology named Fritz Zwicky was studying a cluster of galaxies 300 million light years away, and spotted something curious.
Through the giant 100in Hooker Telescope, Zwicky saw how the galaxies were moving so quickly that they should have been flung into the wider universe like giant cosmic snowballs. Yet, something mysterious, unseen and evidently powerful somehow kept the cluster magically glued together.
The physicist suggested that the only thing that could explain this phenomenon was the existence of some kind of vast, invisible mass that exerted a gravitational pull strong enough to bind the galaxies. In a 1933 paper, Zwicky called this unseen substance Dunkle Materie — dark matter.