What is the Big Bang Theory?

The Big Bang Theory is a well-established cosmological model that explains the origin of the universe. This theory posits that the universe began as an extremely hot, dense point approximately 13.8 billion years ago and has been expanding ever since. The model is supported by several key pieces of evidence, including the observed redshift of galaxies, which indicates that the universe is still expanding, and the cosmic microwave background radiation, which is the afterglow of the initial explosion.

By focusing on the universe's inception and the subsequent expansion, the Big Bang Theory provides a framework for understanding not only how the universe came into existence but also how it has evolved over time. This differs from the other choices, which focus on aspects such as star and planet formation, galactic lifecycle, or dark matter, none of which address the overarching origin of the universe that the Big Bang Theory specifically describes.