The theory is, if someone from the future goes back and change the history, it'll not change the future. Instead it'll create new future branching from the original one. Theoratically there's alternate universe that occupy the smae space we are in right now, but on a different frequency. For example, there could be an alternate universe where the meteor missed the earth and the Dinosaur still walk the earth. Perhaps there's Dinosaur that walk right beside you right now but in alternate reality.