The current understanding in cosmology is that the universe does not expand into any pre-existing space. Instead, the expansion of the universe refers to the increasing distances between galaxies and other cosmic structures. It is the metric of spacetime itself that is expanding.
The concept of the universe having a finite or infinite spatial extent is still an open question in cosmology. Observations suggest that the universe appears to be spatially flat on large scales, indicating that it could be infinite or at least much larger than the observable universe.
If the universe were indeed expanding into a limited space or had a boundary, and the expansion was extremely rapid, it could potentially lead to gravitational interactions between galaxies, stars, and planets. However, it's important to note that the expansion of the universe is not driven by the motion of individual objects within it. Instead, it is the result of the overall expansion of spacetime itself, as described by the cosmological models.
In our current understanding, the expansion of the universe is occurring on such vast scales that individual objects like galaxies and star systems are not experiencing noticeable effects from the expansion. On smaller scales, the gravitational interactions between objects dominate, and they remain relatively unaffected by the cosmological expansion.
It's worth noting that the rate of expansion, as described by the cosmological models, is not directly related to the motion of galaxies within galaxy clusters or other local structures. These structures can have their own internal dynamics and interactions governed by gravity.
It's important to remember that our understanding of the universe is continually evolving as new observations and theoretical advancements are made. The nature of the universe's expansion and its potential boundaries or limitations are areas of active research in cosmology.