The origin of the universe is described by the Big Bang theory, which proposes that the universe began as a hot, dense state approximately 13.8 billion years ago and has been expanding and cooling ever since. This theory is supported by observational evidence such as the cosmic microwave background radiation and the large-scale structure of the universe. While the Big Bang theory provides a framework for understanding the early universe, questions about what triggered the Big Bang and what came before it remain unanswered and are areas of ongoing scientific inquiry.