The current understanding of the expansion of the universe, based on observations and theoretical models, suggests that the universe is not expanding into any pre-existing space. Instead, it is the fabric of space itself that is expanding. This means that there is no "space" outside the universe into which it expands.
To visualize this, imagine the universe as the surface of an inflating balloon. As the balloon inflates, all points on its surface move away from each other. However, there is no need for the balloon to expand into any additional space outside itself.
In this model, galaxies, stars, and planets do not crash into each other due to the expansion of the universe. The gravitational forces between celestial objects, such as stars and galaxies, are much stronger than the relatively weak effects of the universe's expansion. These gravitational forces keep celestial objects bound together within structures like galaxies and prevent them from collapsing or colliding on large scales.
It's important to note that the expansion of the universe primarily affects the large-scale structure of the cosmos. On smaller scales, gravity dominates and holds objects together. While galaxies within galaxy clusters can experience relative motion due to the expansion, the forces of gravity usually keep them gravitationally bound.
However, it's worth mentioning that there are other scenarios and theories that propose different ideas about the nature of the universe and its expansion. Our current understanding is based on the most widely accepted model, known as the Lambda-CDM model, which includes dark energy as the driving force behind the accelerating expansion of the universe.