What is the universe expanding into?

If there was no time, space, or matter before the big bang, and the universe is still expanding and accelerating, what is the universe expanding into? Is it just a big nothing? I don't know about any other budding astronomers, but I don't like the word nothing. No matter where we look there's always something. I need someone else's perspective or opinion on this because I'm writing a paper. I won't use your words in the paper, but I just need a fresh perspective on it. Is it another brane with another universe with different laws of physics, as in M-Theory? Or is it something else. This is the most interesting part of astrophysics to me, and I'm really intrigued about it.