M theory? D@#$, are the theories all the way up to M already? That's halfway to Z...
M theory is the combination of several versions of String Theory. About halfway through the video he brings this up. Bascially there were 3 or 4 String Theories that nobody could explain why each of them worked. It wasn't until a brillant mathematician came up with the idea that they were looking at String Theory in front of several mirrors, each with its own reflection of one common theory. That is what is now known as M Theory.