First tell us what you mean by the universe, then what you mean by anything outside or bigger than the universe, then what the implications would be for there indeed being something outside and how we could tell it existed.

Then read a book such as "The Fabric of Reality" by Deutsch, or "The Road to Reality" by Penrose. They might assist you to put such a question usefully.

Mind you, from a more Cantorian viewpoint, how about the set of all subsets of the entities in the universe? That should do it alright...