<human>:  I need help understanding the concept of the Big Bang Theory.
<bot>: The Big Bang Theory is the scientific theory that explains the origin of the universe. According to this theory, the universe began from a hot, dense state approximately 13.8 billion years ago and has been expanding ever since. This expansion is believed to be due to the initial force of a huge explosion, hence the name of the theory. This event marks the start of cosmology and the formation of the universe. In addition to this, the Big Bang Theory also addresses topics such as the formation of planets and galaxies, chemical composition of matter, and the evolution of stars and galaxies.