If the universe is expanding, whats it expanding into

Edit: Some scientist might tell you they don't understand what it is expanding into, but that would be because they are biologist or chemists. Physicists aren't having that problem. They know it isn't expanding into anything, and how something that is expanding can be expanding without expanding into something. It isn't unanswerable. Merely hard if one doesn't put a small amount of effort into it. I understand it, and I'm a fracken' idiot.