The concept of the universe expanding can be a bit counterintuitive, but I'll do my best to explain it.
When scientists say that the universe is expanding, they mean that the distances between galaxies and other cosmic structures are increasing over time. It's not that the universe is expanding into something outside itself; rather, it is the fabric of space itself that is expanding.
To understand this, imagine a two-dimensional surface, like the surface of a balloon. If you draw dots on the balloon and then inflate it, the dots will move away from each other as the balloon expands. The surface of the balloon itself represents the universe, and as it expands, the galaxies and clusters of galaxies move apart.
This expansion of the universe was first proposed by the Belgian astronomer Georges Lemaître and later supported by observational evidence, most notably by Edwin Hubble's observations of galaxies in the 1920s. Hubble found that galaxies were generally moving away from each other, indicating that the universe was expanding.
It's important to note that the concept of the universe expanding does not necessarily imply that there is something "outside" the universe into which it is expanding. The universe is considered to be all-encompassing, and the expansion refers to the stretching of the fabric of space itself.
However, it's worth mentioning that the nature of the universe and its expansion are active areas of scientific research, and there are different theories and models being explored to better understand these phenomena.