Astronomy
Galactic Rotation Curves and Dark Matter Evidence
Quick fact
The outer regions of galaxies rotate at speeds so high that they should fly apart into intergalactic space—unless there is about five times more mass than what we can see.
Why this is interesting
You've probably seen swirling spiral galaxies, and you might assume their stars orbit slower far from the center. But measurements show the outskirts spin just as fast as the inner parts—defying all expectations. What invisible hand is holding them together?