The Big Bang Theory is the leading explanation for how the universe began. It describes the universe as we know it starting with an infinitely hot and dense single point that inflated and expanded. The theory explains the evolution of the observable universe from the earliest moments after the Big Bang. The Big Bang models offer a comprehensive explanation for a broad range of observed phenomena, including the abundances of the light elements, the cosmic microwave background radiation, large-scale structure, and Hubbles law. The models depend on two major assumptions: the universality of physical laws and the cosmological principle. The universality of physical laws is one of the underlying principles of the theory of relativity. 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 theory is supported by a wide range of observational evidence, including the cosmic microwave background radiation, the abundance of light elements, and the large-scale structure of the universe.