The most commonly accepted theory about the origin of the universe is the Big Bang Theory. This theory posits that the universe began as a singularity approximately 13.8 billion years ago and has been expanding ever since. During the initial moments, all matter and energy were concentrated in an incredibly hot and dense state, leading to rapid expansion and cooling. As the universe expanded, matter began to form stars and galaxies. The Big Bang Theory is supported by various observational evidence, including the cosmic microwave background radiation and the redshift of distant galaxies.