String theory
From The Big Bang Theory Wiki
String theory is a growing part of theoretical physics that puts together quantum mechanics and general relativity into a quantum theory of gravity. A quantum theory of gravity is very important, because it would describe the entire universe at once ... more from Wikipedia
Sheldon spends a good deal of his professional time theorising about string theory (esp. 0101, 0112, 0202, 0217, 0223). At one point, he thought that his expedition to the North Pole in search of magnetic monopoles (0223) had proved string theory, only to discover that his data had been manipulated (0301).
