Anonymous asked: Are there words out there that mean an entire imagined world inside of your head? Sort of like brainchild, but more vast?

Sort of. I have paracosm, a detailed imaginary world that often features people, animals, or alien creatures, but it pretty specifically refers to such an imaginary world that is created by a child or young person. I don’t have a word that’s for a world created by adults. I’m sorry! That’s the best I’ve got.