Don't go looking for a diploma in the Batcave. The Dark Knight's alter ego left Gotham City for a stint at Princeton University, but as "Batman Begins" fans know, he returned home and then embarked on a very different kind of study abroad. "No one has pointed out the most important thing about Bruce Wayne, which is, yes, he attended Princeton, but he didn't graduate," "Dark Knight" trilogy director Christopher Nolan told Princeton's Class of 2015 on Monday.