The universe's origins are commonly explained by the Big Bang theory, which suggests that around 13.8 billion years ago, all matter and energy were concentrated in a single point. This point then rapidly expanded, leading to the formation of space, time, and the fundamental forces of nature. As the universe cooled, particles began to form, eventually creating atoms. Over millions of years, these atoms clumped together under the force of gravity, forming stars and galaxies. The first stars ignited nuclear fusion, producing heavier elements, which later contributed to the formation of planets and other celestial bodies, shaping the universe as we know it today.