Don Lincoln | TED-Ed
View full lesson: http://ed.ted.com/lessons/dark-matter…
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.