And now we've come full circle because my question was what is the universe expanding into?
The way I see it, the universe isn't expanding
into anything. All the spatial and temporal dimensions wrap around themselves and form closed loops, and when the universe expands the loops get bigger. I suppose "expand" isn't quite the right word; "warp" is more like it, since the dimensions warp in such a way that they can contain more matter at a fixed density.
Sigh... I'm bad at explaining this. Let me use another analogy. Imagine that you are an AI inside a computer program, a simulated world of sorts. What lies outside of the computer? Since your existence is entirely defined by the computer program, the outside of the computer holds no meaning to you. For all intents and purposes, the universe doesn't have an outside.
And now you're going to say that science can't answer your question, but God can. Sure, God can ignore the rules and do anything, while science can't. There is no way we can win. In a way that makes sense, since science is limited by our perceptions; perhaps God is the truth or reality behind the facade of perception. I believe that Infinity is this truth, so I suppose that makes me a scientific theist of sorts; but I certainly do not believe in a single word of Christianity.