The Milky Way's Rotation Curve: The Dark Halo Puzzle
Think of a merry-go-round: the farther you sit from the center, the faster you're carried around. For a long time, astronomers assumed galaxies work the same way — stars at the edges should orbit more slowly than those near the center. But back in the 1970s, it turned out that in many spiral galaxies, the speed of stars barely drops with distance from the center.