That's the confusing part. You're mind has a terrible time with "nothing." Nothing doesn't mean empty space, it means nothing. There isn't anything to expand into. Even if there was it's impossible for us to know anything about it, and it's impossible for it to have any influence on our physical universe unless the speed of light is not constant.
Rather of thinking of it as "nothing" I would rather view it as "devoid of anything" as it is easier to wrap your head around, but yeah, even then it is impossible for us to understand the concept of "nothingness".
I have this weird theory where the universe isn't expanding so much as it is being "sucked" from the outside, like how a ballon inflates inside a low air pressure room.
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