The Big Bang is a scientific theory about how the universe started, and then made the stars and galaxies we see today. The universe began as a very hot, small, and dense superforce (the mix of the four fundamental forces), with no stars, atoms, form, or structure (called a "singularity") Definition of big bang theory. : a theory in astronomy: the universe originated billions of years ago in an explosion from a single point of nearly infinite energy density — compare steady state theory. As space expanded, the universe cooled and matter formed. One second after the Big Bang, the universe was filled with neutrons, protons, electrons, anti-electrons, photons and neutrinos. During the first three minutes of the universe, the light elements were born during a process known as Big Bang nucleosynthesis.