Pop physics

Ok so more dark matter. Last time I told you about the guy who first came up with the idea, Fritz Zwicky. But like I said he was kinda crazy so no one took him seriously. It took experimental evidence to get the idea under way again. In the 1970’s this lady named Vera Rubin looked at the rotational velocity of stars in different places throughout the galaxy and found the graph that I’ve included. So basically she looked at the speed of a star near the center and then a star further out in the galaxy and what we expect to see is a steady drop in speed as we go further out because the gravity is weaker out there. BUT what she found was that the stars near the edge of the galaxy go just as fast as the stars near the center. But this doesn’t make any sense with the amount of gravity we thought there was. So what is the explanation? There must be some more gravity that we can’t see. There must be dark matter!!!!