The concept of the Big Bang does not imply the existence of a pre-existing universe. Instead, the Big Bang theory suggests that the universe originated from an extremely hot and dense state, often referred to as a singularity, nearly 13.8 billion years ago. According to this theory, all matter, energy, space, and time that make up the universe today were compressed into an infinitely small point.
The Big Bang theory is supported by a range of observational evidence, such as the cosmic microwave background radiation, the abundance of light elements, and the observed expansion of the universe. These pieces of evidence indicate that the universe has been expanding and evolving since its early stages.
However, the Big Bang theory does not provide an explanation for what caused the initial singularity or what, if anything, existed before it. It describes the evolution of the universe from an extremely hot and dense state onward. The origin of the universe and the nature of the singularity itself remain open questions in cosmology, and scientists continue to explore different hypotheses and theories to better understand the early moments of our universe.