There's a lot of unexplored territory in the Marvel Cinematic Universe between Thanos snapping his fingers in Avengers: Infinity War and the main events of Endgame five years later. As the latter movie explains, Bruce Banner spent at least 18 months of that period experimenting with gamma radiation, which allowed him to successfully merge his consciousness with Hulk's physical form. The question is: what did he do during the remaining 42 months?
A post on the Marvel Studios subreddit theorizes that Hulk refashioned himself as a science celebrity following The Snap and even launched his own educational YouTube Science channel for elementary school children. The fan theory is primarily based on the scene in Endgame where Hulk, while meeting with the remaining Avengers to discuss using time-travel to undo Thanos' actions, is approached by some young fans. He proceeds to pose for a photo with them, spouts his new catchphrase ("Hulk out!") and tells them to "Listen to your mom." In other words, he acts like a dorky school teacher, even dabbing at one point.
The theory speculates the actual series is called Professor Hulk so it's worth pointing out the kids in this Endgame scene refer to Hulk as "Mr. Hulk," not "Professor Hulk." Then again, Hulk could still have his own Bill Nye the Science Guy-style show that doesn't identify him as a professor, leaving room for the kids to address him this way. Something similar could be said for his "Hulk Out" catchphrase, seeing as it doesn't necessarily sound like something an online science instructor would say. On the other hand, it doesn't take a huge leap to imagine it could come to refer to the idea of being smart and studious like the Hulk, in light of his new and improved public image.
As delightful as the idea is of Hulk being a goofy and awkward science expert who teaches children, there's not really anything in Endgame that supports this theory, outside of the cringe-worthy dialogue in the aforementioned scene. However, there isn't anything that necessarily contradicts the idea, either. Furthermore, it's clear throughout Endgame that Hulk has completely lost his taste for violence and prefers to avoid fighting unless it's absolutely necessary. The thought of him spending his days trying to help rebuild society by educating kids following The Snap would make much more sense than him devoting his efforts to fighting crime on his own.
There is a deleted scene from Endgame that shows Hulk leaping over rooftops to rescue some citizens who are trapped in a burning building, but even that doesn't rule out the possibility of Hulk teaching children on YouTube in between the occasional act of super-heroism. In fact, it would also fit with the idea that Hulk swore off violence after he and Banner merged, and from then on only tried to better society in other, more peaceful ways.
Still, if Mark Ruffalo reprises his role as Hulk during the upcoming She-Hulk series on Disney+, as he's reportedly in talks to do, maybe the show will take this theory all the way home and make the Professor Hulk YouTube series MCU canon. Wilder things have happened, right?
Directed by Joe and Anthony Russo, Avengers: Endgame stars Robert Downey Jr. as Iron Man, Chris Evans as Captain America, Mark Ruffalo as Bruce Banner, Chris Hemsworth as Thor, Scarlett Johansson as Black Widow, Jeremy Renner as Hawkeye, Brie Larson as Captain Marvel, Paul Rudd as Ant-Man, Don Cheadle as War Machine, Karen Gillan as Nebula, Danai Gurira as Okoye and Bradley Cooper as Rocket, with Gwyneth Paltrow Pepper Potts, Jon Favreau as Happy Hogan, Benedict Wong as Wong, Tessa Thompson as Valkyrie and Josh Brolin as Thanos.