The whole thing seems like a sort of mental masturbation to me. How do you define the universe? Usually I take it to mean all that which we are able to detect. If we can detect something, it is part of the universe. Therefore, anything 'outside' the universe is by definition, undetectable and can be considered to not exist. As subjective beings, the only things that really matter are those that we can experience, so if something is intrinsically beyond our experience, then it effectively is non-existent.

The multiverse idea is not the same as something 'outside' the universe in my opinion. Universe means 'all that is' so technically a different time-space continuum that runs parallel to our own is still part of the one universe. This may just be a problem with labelling, of course, since no matter how much stuff we find, all of it together will still be contained in the universe simply by how the word is defined.