The concept of what lies "outside" the universe is a complex and philosophical question that does not have a definitive scientific answer at present. The expansion of the universe, as described by the Big Bang theory, refers to the stretching of space itself. It does not necessarily mean that the universe is expanding into something or expanding within a preexisting space.
One way to think about it is by considering the universe as a two-dimensional surface of a balloon. As you inflate the balloon, all points on its surface move away from each other, similar to how galaxies and celestial objects move apart in the expanding universe. However, it's important to note that this analogy is not perfect since the universe is not expanding into anything in the same way that the balloon is expanding into the surrounding space.
The current understanding is that the expansion of the universe is happening on a cosmological scale, affecting the distances between galaxies and clusters of galaxies. Beyond that, it becomes difficult to ascertain what lies beyond our observable universe or what the true nature of "nothing" might be.
It's worth mentioning that the concept of what lies outside the universe is still an area of active scientific investigation and speculation. Theoretical physicists and cosmologists propose various ideas, such as the existence of multiple universes or a multiverse, but these ideas remain theoretical and speculative at this point.