The concept of the expansion of the universe can be a bit counterintuitive, and it is important to note that the expansion is not a movement of galaxies or objects within the universe through preexisting space or into something beyond the universe. Instead, it refers to the stretching of space itself.
According to the prevailing Big Bang cosmological model, the expansion of the universe means that the distances between galaxies and other cosmic objects are increasing over time. This expansion can be visualized by imagining dots on the surface of an inflating balloon. As the balloon inflates, the dots move away from each other, even though they are not moving within the surface of the balloon itself.
As for what the universe is expanding into or what was there before the expansion, our current understanding is limited. The expansion of the universe is described mathematically using the framework of general relativity, which focuses on the dynamics of space and time within the universe itself. It does not provide an explanation for what might exist or have existed beyond the observable universe.
Some speculative ideas in cosmology, such as the multiverse hypothesis, propose the existence of other universes or additional dimensions beyond our observable universe. However, these ideas are still highly speculative and lack direct observational evidence.
In summary, the expansion of the universe refers to the stretching of space itself, and it is not expanding into anything as commonly understood. The question of what the universe may be expanding into or what existed before the expansion remains an open and active area of scientific research and speculation.