The concept of the expansion of the universe can be a bit counterintuitive, but I'll try to explain it as best as we currently understand it based on scientific theories and observations.
When scientists say that the universe is expanding, they are referring to the idea that the distances between galaxies are increasing over time. However, it's important to note that the expansion is not happening within some pre-existing space or material that the universe is expanding into. It is the space itself that is expanding.
The universe, as we currently understand it, is all-encompassing. It includes all matter, energy, and the fabric of space itself. So when we say that the universe is expanding, we mean that the space between galaxies, clusters of galaxies, and other cosmic structures is getting larger.
To visualize this, imagine dots on the surface of an inflating balloon. As the balloon is inflated, all the dots move away from each other, and the space between them increases. However, there is no need for the balloon to expand into anything because the surface of the balloon itself is all there is.
Similarly, the expansion of the universe is the stretching of space itself. It is not expanding into any external or pre-existing space. The question of what lies beyond the observable universe or whether there is something "outside" the universe is still a topic of scientific investigation and speculation. However, our current understanding suggests that our observable universe is a finite region of a much larger, possibly infinite, universe, and we have limited information about what may lie beyond our observable boundaries.
It's important to note that these concepts are based on our current understanding of the universe, which is continually evolving as scientific research progresses.