View full lesson: http://ed.ted.com/lessons/dark-matter-how-does-it-explain-a-star-s-speed-don-lincoln
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.
Lesson by Don Lincoln, animation by RocknRoll Animation.