What theory suggests that the universe began with a massive explosion?

The Big Bang Theory is the explanation that the universe began from an extremely hot and dense state and expanded rapidly, which is often envisioned as a massive explosion. This theory is supported by a multitude of astronomical observations, such as the cosmic microwave background radiation, which is the remnant heat from the initial expansion, and the redshift of galaxies that indicates they are moving away from us, suggesting that the universe is still expanding. The Big Bang Theory provides a comprehensive framework for understanding the origin and evolution of the universe, including the formation of galaxies, stars, and all matter, and it marks a significant shift in how we understand cosmology compared to earlier models like the steady-state theory, which posits that the universe has always existed in a constant state without beginning or end.