So back in the 1930's, astronomers noticed that there didn't seem to be enough mass in galactic clusters to hold them together, but they were held together. There mas be some additional mass and gravity to hold everything together, but where is it? It can't be seen. And this problem produced the theory of dark matter. Follow Cosmic Coffee Time on Twitter for some special content twitter.com/CosmicCoffTimeYou can request a topic for the show! Email it to cosmiccoffeetime@gmail.com