<human>:  Please explain the concept of the big bang theory.
<bot>: The Big Bang Theory is the prevailing cosmological model for the universe from the earliest known periods through its subsequent large-scale evolution. The model describes how the universe expanded from a very high-density and high-temperature state, and offers a comprehensive explanation for a broad range of phenomena, including the abundance of light elements, the cosmic microwave background (CMB), large scale structure and Hubble's law (the observation that the universe is expanding). According to the Big Bang theory, the universe was originally in an extremely hot and dense state and then expanded rapidly. It continues to expand today, and the observation of this expansion is known as Hubble's Law. As the universe expands, the density of matter within it decreases, while energy density remains constant. This means that the universe is cooling as it expands over time, and as it cools, matter clumps together to form stars, galaxies, and other structures.