TED-Ed

Dark matter: How does it explain a star's speed? - Don Lincoln (2012x79)

Air date: Nov 12, 2012

All the stars in a spiral galaxy rotate around a center -- but to astronomers, the speed that each star travels wasn't making sense. Why didn't stars slow down toward the edges as expected? Don Lincoln explains how a mysterious force called dark matter is (possibly) the answer -- and why the search for an answer matters.