The Big Bang is a physical theory that describes how the universe expanded from an initial state of high density and temperature. It is the leading explanation for how the universe began. The theory states that the universe started as a single point, known as a singularity, which was infinitely hot and dense. Suddenly, the singularity began expanding, and the universe as we know it began. As the universe expanded, its temperature and density decreased, and atoms formed, then molecules. Gravity drew the matter into greater and greater clumps, eventually bringing into existence the stars, planets, and galaxies we see today. The Big Bang theory does not describe how energy, time, and space were caused, but rather it describes the emergence of the present universe from an ultra-dense and high-temperature initial state. The name "Big Bang" is a misnomer because it evokes an explosion, which is not an accurate description of the event. The Big Bang only describes the intrinsic expansion of the contents of the universe. The universe is 13.8 billion years old, and the Big Bang is the point in history where the universe entered into a regime where the laws of physics as we understand them work.