The Animation is the name toons give to the parallel universe they come from. It has intersected with Hollywood to create Toon Town, a small city that partially exists on both sides of the dimensional divide. Unlike many universes, the Animation is literally infinite in terms of both size and energy. It may be connected to certain humans subconsciously, allowing their artistic ideas to flow in, or perhaps seeding their mind with visions of what already exists.
As the Animation has infinite space, it also has infinite time, which means it has always existed and will continue to always exist. There may still have been a time, from a perspective outside the Animation, where it did not always exist. But there is no way to accurately tell, as it exists now and therefore always has and always will. Most scientists have invested zero time and resources into investigating the Animation, citing it as a waste of time, but a few fringe researchers have put forward theories with absolutely no basis in anything.
The Living Subconscious theory postulates that the Animation is formed from the undirected thoughts of the entirety of humanity, reflecting the way humanity sees itself through a warped lens of interference in the form of deep-rooted psychological issues. The major flaws with this theory are that the toons of the Animation have managed to retain a positive attitude and a desire to make others happy despite prolonged contact with humanity in general and Hollywood specifically.
The Wizard Did It theory postulates that the Animation is an artificial creation brought into existence by "an powerfulle wizzard"[sic] and that "his majickal powerse"[sic] have circumvented "manne's science"[sic] in order to create pure and limitless energy. There are a number of minor flaws with this theory, but so far it is the most promising.
The True World theory postulates that the Animation is the only "real" reality in existence, and all others are incomplete or otherwise broken imitations of it. This theory has been dismissed as too depressing to think about.
The Animation operates on laws of physics that are entirely separate and mostly-incompatible with Hollywood's own. Toon physics gives the illusion of being similar, but the underlying processes are wildly variable and depend upon personal interpretation. A common example is gravity: objects will fall as fast as they like, only when they observe themselves to be capable of doing so. A toon can happily walk across a chasm without altering their trajectory in the slightest, unless they notice where they are walking.
All entities within the Animation, including those not native to the Animation, operate under these physics. This means that visiting humans from Hollywood can stretch, defy gravity, and cheat death just as easily as any toon. However, while toons retain access to these physics outside of the Animation through the perpetual energy link that sustains them, the same is not true for those not native. Multiple casualties have occurred when humans have come back from visits to Toon Town without first ensuring they are the right shape and free from any arrows, anvils, bullet holes, or lit sticks of dynamite.