Like to the "if God is the prime mover who caused God?" question there is no answer to this matryoshka question.
Of course we imagine the universe like a ballon that is blown up but our imagination fails here. There is nothing outside of the universe so it is not expanding into anything. Our primate brains have not evolved to imagine this kind of stuff, that's why it will always be totally abstract.
It's like when theoretical physicists say that they cannot get their head around quantum mechanics. They might understand their model but not the phenomenon in and of itself. For us the very tiny and the very large break down into mathematical abstractions and are not really perceivable.