The formation of the universe is primarily explained by the Big Bang Theory, which posits that the universe began as an extremely hot and dense point approximately 13.8 billion years ago. As it expanded, it cooled, allowing matter to form and eventually leading to the creation of galaxies, stars, and planets. Over time, the universe continued to evolve, with structures forming under the influence of gravity. The Cosmic Microwave Background radiation, a remnant of the early universe, provides crucial evidence for this formation process, helping scientists understand the conditions that existed shortly after the Big Bang.