There have been plenty of crossover events between the Marvel and DC Comic universes over the years, but it may be a third outside property that most intrinsically ties together the two comic book powerhouses: The Teenage Mutant Ninja Turtles.
It's a poorly-kept secret that the Ninja Turtles were sneakily meant to share the same origin story with Matt Murdock's Daredevil. The chemical that blinded young Matt was meant to be the very same Ooze that turned everyone's favorite baby turtles in the Heroes in a Halfshell fans have loved for the past few decades. And the Daredevil/Marvel connections and homages don't end there. While Matt Murdock retrieved his training from an older master called Stick, the Turtles had Splinter. Instead of constantly facing off with Daredevil's The Hand, the Turtles' nemesis is The Foot Clan. TMNT creators Kevin Eastman and Peter Laird were gloriously unsubtle about it all.
So if the Ninja Turtles technically sort of exist within the Marvel Comics universe, that means whatever other universes they pop up in are also arguably entwined. So, say, when the reptilian heroes teamed up with Batman, could a conclusion be drawn that the Turtles exist in both the Marvel and DC Universes? The Turtles have fought side-by-side with the Power Rangers (with TMNT arch nemesis Shredder even once became the Green Ranger), so that drags another franchise into the mix. The Ghostbusters apparently share the same universe, considering they've teamed up with the Turtles on more than one occasion. So does that mean Angel Grove, CA exists in the same world as Gotham and Wakanda? Does Pete Venkman drive past Avengers Tower on his way to work in the Ghostbusters' New York? The connection, tentative though it may be, opens a can of worms in terms of potential fan theories.
Extrapolating further as to how many different fictional universes the Turtles tie together, and the various and exponential branches those connections create is a head-spinning exercise in nerdiness. The Ghostbusters have crossed paths with the Mars Attacks aliens, so there's another fictional world to add to the mix. The Punisher and Archie have crossed paths multiple times, so does that mean Riverdale exists in this universe? As a matter of fact, even Batman has met Archie in the past, adding slightly more credence to this admittedly ridiculous idea. The Justice League and the Power Rangers have co-mingled, offering even more "proof."
Following this absurd logic and diving deeper down the rabbit hole, the possibilities are almost endless. Chun-Li once became a Power Ranger, so that drags the Street Fighter universe into the mix. The Turtles have crossed over into the X-Files universe, which coexists with The Simpsons universe. Predator, Aliens, and even Sharknados could all exist within this weird, ever-expanding hypothetical universe. The number of connections to be drawn is truly staggering enough to bust out a cork board and some red strings, because there's a Pepe Silvia-level of connections to be made. Add in the fact that Michelangelo appeared in the '80s anti drug PSA movie Cartoon All Stars to the Rescue, and the mind will boggle with interconnecting universes.
Is there honestly any credence to the theory that the Teenage Mutant Ninja Turtles alone are responsible for uniting most every pop culture universe in the world? Of course not. Obviously this is all just fun fan speculation to be taken with a heaping pile of salt. Comic book character and franchise crossovers are often akin to a kid grabbing their favorite action figures at random and smashing them together, developing a new fictional universe out of whole cloth. But what's the point of pop culture fandom if not to explore the limits of one's imagination? If nothing else, all this proves that creators enjoy testing those limits and playing with their toys just as much.