"How do we know that dark matter is there?"  In this episode, Dr. Katie Mack and John Green unpack the full story of dark matter.Head to https://policygenius.com/crashcourse to get your free life insurance quotes and see how much you could save.This show is a production of Complexly. If you want to help keep Crash Course free for everyone, forever, you can join our community on Patreon at  http://www.patreon.com/crashcourse