The Big Bang theory is the prevailing cosmological model for the earliest known periods of the universe and its subsequent large-scale evolution. It states that the Universe was in a very high density state and then expanded. If the known laws of physics are extrapolated beyond where they are valid there is a singularity. Modern measurements place this moment at approximately 13.8 billion years ago, which is thus considered the age of the universe. After the initial expansion, the Universe cooled sufficiently to allow the formation of subatomic particles, and later simple atoms. Giant clouds of these primordial elements later coalesced through gravity to form stars and galaxies. The Big Bang theory does not provide any explanation for the initial conditions of the Universe; rather, it describes and explains the general evolution of the Universe going forward from that point on. Since Georges Lemaître first noted, in 1927, that an expanding universe might be traced back in time to an originating single point, scientists have built on his idea of cosmic expansion. While the scientific community was once divided between supporters of two different expanding universe theories, the Big Bang and the Steady State theory, accumulated empirical evidence provides strong support for the former. In 1929, Edwin Hubble discovered indications that all galaxies are drifting apart at high speeds. In 1964, the cosmic microwave background radiation was discovered, which was crucial evidence in favor of the Big Bang model, since that theory predicted the existence of background radiation throughout the Universe before it was discovered. The known physical laws of nature can be used to calculate the characteristics of the Universe in detail back in time to an initial state of extreme density and temperature.