In the 1960s and 70s, astronomers measured the speed of stars and gas clouds orbiting at the edge of our galaxy, and discovered something strange: their rotational velocities were too fast. Way too fast. So fast, there’s no way the gravity from the visible mass in the galaxy could possibly keep them in orbit.
There was clearly some mass out there across the galaxy that astronomers couldn’t see. Like, more than three quarters of it. Awkward!
Astronomers call this invisible stuff Dark Matter, and fro decades the evidence has mounted for its existence across the universe. But in March 2018 astronomers found a galaxy with no Dark Matter at all … which, in a wonderful leap of logic, gives them even more confidence that this mysterious material exists.