The Big Bang theory is the most widely accepted explanation for the origin and evolution of the universe based on extensive observational evidence and theoretical frameworks in cosmology. It describes the expansion of the universe from a highly dense and hot state to its current state over the course of billions of years.
While the term "Big Bang" might evoke the idea of an explosion in the conventional sense, it is important to note that the Big Bang was not an explosion that occurred within preexisting space. Instead, it refers to the rapid expansion of space itself. The early universe was in a highly compressed, hot, and dense state, and as space expanded, it carried matter and energy with it, resulting in the expansion and cooling of the universe.
Regarding the question of what caused the Big Bang or what existed before it, our current understanding has limitations. The Big Bang theory describes the evolution of the universe from an extremely early stage, often referred to as a singularity, where our current models of physics break down. It is difficult to make definitive statements about what caused the Big Bang or what existed prior to it using our current understanding of physics.
However, it is worth noting that the Big Bang theory does not claim that the universe emerged from "nothing" in the philosophical sense. It focuses on the evolution of the universe from an extremely hot and dense state. Some physicists and cosmologists propose theoretical ideas, such as inflationary cosmology, to explain the initial conditions and the expansion of the universe, but these ideas are still under investigation and are subject to ongoing scientific research.
Overall, the Big Bang theory is the prevailing scientific explanation for the origin and development of the universe based on the evidence available to us. However, the precise nature of what caused the Big Bang and what existed before it are still active areas of research and remain topics of exploration within the scientific community.