The concept of the expansion of the universe can be a bit counterintuitive, so let's delve into it further.
When we say that the universe is expanding, we mean that the distances between galaxies and other cosmic structures are generally increasing over time. It's important to note that the expansion of the universe is not a matter of galaxies moving through space but rather the underlying fabric of space itself stretching.
However, the expansion of the universe does not imply that there are places the universe has yet to reach. The universe, by definition, encompasses all of space and time. It's not a case of the universe expanding into some pre-existing space; rather, the expansion is happening within the framework of the universe itself.
To put it simply, as the universe expands, the distances between galaxies and other celestial objects grow larger. It's not that the universe is expanding into a pre-existing void; rather, space itself is expanding, carrying galaxies along with it. The expansion is happening everywhere simultaneously, and there is no "before" the universe gets to a certain place because the universe encompasses all of space and time.
It's important to note that the concept of the expansion of the universe is based on our current understanding of cosmology and observational evidence, primarily from measurements of the cosmic microwave background radiation and the redshift of distant galaxies. However, the nature of the universe and its expansion are still active areas of research, and scientists continue to study and refine our understanding of these complex phenomena.