Why didnt stalin join forces with hitler during world war 2

Stalin joined the Allies in World War Two because Hitler had invaded the USSR.

While he and Hitler may have had the same attitude towards killing innocent civilians, Josef Stalin was the Soviet Union's leader during World War II and ended up against Hitler when he tried to invade the Soviet Union. Stalin soon joined the Allied Powers after Hitler tried to invade. Hitler was extremely cruel to Soviet P.O.W. (Prisoners of War) and pretty much starved them to death. They were mortal enemies. 

The big bang theory suggests that the universe is constantly expanding and that stars, galaxies and other entities are moving away from each other. if there were a galaxy moving closer to earth then that would contradict the big bang theory since the entities should be moving away from earth and from each other.