The universe started approximately 13.8 billion years ago with an event known as the Big Bang. This theory, which is the most widely accepted explanation, proposes that the universe began as an extremely hot, dense state that was not confined to a single point but rather was the state of space itself at that moment. The universe then rapidly expanded and cooled, allowing the formation of subatomic particles, then atoms, and eventually stars, galaxies, and other celestial bodies. This expansion of the universe is ongoing and continues to shape the cosmos today.