The Animation
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.
Possible origins
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.