The concept of the Big Bang theory does not imply that it was the end of a previous universe consisting of galaxies and planets. Instead, the Big Bang theory describes the origin and early development of our current universe.
According to the Big Bang theory, the universe as we know it began as an extremely hot and dense state approximately 13.8 billion years ago. At that point, all matter and energy were concentrated in an infinitesimally small and dense singularity. Then, for reasons that are not fully understood, this singularity underwent a rapid expansion known as the Big Bang, leading to the formation of space, time, and matter.
In the early stages of the universe's expansion, matter and energy were in a highly energetic and hot state, and it took some time for the universe to cool down and for matter to form structures such as galaxies, stars, and planets. The formation of galaxies and planetary systems occurred gradually over billions of years as matter clumped together under the influence of gravity.
Therefore, the Big Bang theory describes the beginning of our current universe, not the end of a previous universe. It provides a framework for understanding the evolution of the universe from its early stages to its present state.