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.

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