What is the most expansive fictional universe ever created?

The Tommy Westphall Universe. A long time ago, in a fabled era known as the ’80s, there was a TV show called St. Elsewhere. It was about a run-down teaching hospital named St. Eligius in Boston and the doctors who worked there. Dr. Donald Westphall was the director of medicine, a widower who raised his two children by himself. One of those children was his autistic son Tommy. Tommy only appeared in fifteen episodes of the series. St. Elsewhere ran for six seasons and won eleven Emmys, but all anyone cares about today is its final episode. In the final scene of the final episode, Tommy Westphall holds a snow globe that reveals the building of St. Eligius inside it. And his father, who is not a doctor, comes in and says the following:

I don’t understand this autism thing, Pop. Here’s my son; I talk to him; I don’t even know if he can hear me. He sits in his world all day long, staring at that toy. What’s he thinking about?

The entire six seasons of St. Elsewhere were, in fact, a child’s daydream while looking at a snow globe.

Here’s where things get a little complicated. The character Dr. Roxanne Turner from St. Elsewhere appeared in an episode of Homicide: Life on the Street, where authorities accused her of murder. But if Dr. Turner was just a creation of Tommy, how could she possibly be on Homicide? Unless Homicide was also Tommy’s daydream. There is an episode of St. Elsewhere where the doctors of St. Eligius decide to go out for a few drinks at a local Boston bar. That bar is Cheers, the titular bar from the sitcom Cheers. So, Cheers, and Frasier are again products of Tommy’s imagination.

Detective John Munch was a character played by Richard Belzer, who starred in Homicide: Life on the Street, which we know never existed. After the cancellation of Homicide, the character was moved to Law and Order: Special Victims Unit. Detective Munch also appears in The Wire, The X-Files, and Arrested Development. So, Tommy Westphall had to create all those shows.

Cheers spun off Frasier, who crossed over with Caroline in the City, with Friends, who shared a character with Mad About You, who crossed over with Seinfeld. In a few centuries, the world of Buffy the Vampire Slayer will be Star Trek, but its distant past (sorry, spoilers) is the reboot of Battlestar Galactica. Doctor Who is canonically taking place in the same universe as I Love Lucy, Hannah Montana, Grey’s Anatomy, and All My Children.

All of it is the creation of one child, which probably explains the continuity errors. For example, no one acknowledges the zombie outbreak in Georgia in The Walking Dead, which is happening at the exact same time as It’s Always Sunny in Philadelphia and Arrow.

If you map everything out, there are at least 419 shows that are in the same continuity with each other and canonically the creation of Tommy Westphall.

The first person to propose the Tommy Westphall Universe was legendary writer Dwayne McDuffie in a blog post criticizing comic book continuity. It was about just how silly it was to try to fit vast and mutually incoherent works all into the same rigid continuity. But he was kind of onto something with that whole Tommy Westphall stuff. They did all crossover with each other.

By the way, the Collector in Guardians of the Galaxy has Tobias Fünke on his ship, which means Tommy Westphall is responsible for the MCU.