The Big Bang theory is a widely accepted explanation for the origin and expansion of the universe. It states that the universe began as a singularity about 13.8 billion years ago, and has been expanding ever since. This theory is supported by various scientific discoveries, including the observation of galaxies moving away from each other, calculations estimating the Big Bang to have occurred 13.7 billion years ago, and recent studies suggesting an even older age of 26.7 billion years.