Shared universes go back way further than Tony Stark, Steve Rogers and co sitting down for some post-Chitauri shawarma. Superheroes have been moonlighting in each other's comic books for decades, while Cheers regulars frequently paid Frasier a visit in Seattle. It wasn't until Marvel Studios launched the MCU (Marvel Cinematic Universe), however, that the concept started to gain serious mainstream traction.
By incorporating the contrasting adventures of Iron Man, Captain America, Thor and Black Widow into one gigantic, overarching narrative, Marvel successfully blended cinematic spectacle with the "must-watch-every-episode" ethos of serialised TV. The MCU's famous end-credits stings also had the unexpected side-effect of convincing us all to stick around until the end of the closing titles of every movie, y'know, just in case.
