The Big Bang theory is the leading explanation for the origin of the universe. It suggests that around 13.8 billion years ago, the universe began as an extremely hot and dense point, which then expanded rapidly. This expansion continues today, leading to the vast cosmos we observe. As the universe expanded, it cooled, allowing particles to form and eventually combine into atoms. Over time, these atoms clumped together to create stars, galaxies, and other cosmic structures. The Cosmic Microwave Background Radiation is a key piece of evidence supporting the Big Bang theory, as it represents the afterglow of this initial explosion.