Well, a UNIverse is selfcontaining. There can be nothing "outside" it, as there EXISTS nothing but the real, the real universe. All "else" is nonexistence.
Langan/Wheeler said thus something like this : That it is the feedback loop where two mirrors reflect a beam of light between each other. It's selfcontaining, but the universe has only one mirror. That's the "explanation" of monism.
Some are talking about decoherent multiverses, which is something else than different universes "existing" in nonexistence paralle to each other.