The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. It proposes that approximately 13.7 billion years ago, the Universe was in an extremely hot and dense state and has been expanding ever since. The theory developed from observations showing that galaxies are moving away from each other and theoretical considerations from Einstein's equations. Measurements of the cosmic microwave background and other factors strongly support the ΛCDM model and provide an estimated age of the Universe of 13.75 billion years according to the Big Bang theory.