Astronomy
Evidence for Dark Matter from Galactic Rotation Curves
Quick fact
In the 1970s, astronomer Vera Rubin measured galaxy rotation curves and found them flat out to the limits of observation, contradicting the expected Keplerian drop-off by a factor of at least five.
Why this is interesting
Imagine spinning a bucket of water—the faster you spin, the harder the water pushes outward. Now picture stars orbiting a galaxy: the outermost stars should move slower, but observations show they race along just as fast as inner stars. Why aren't they flying away?