The Big Bang was not an explosion at a point in space, but the beginning of space itself expanding everywhere at once. Within fractions of the first second the fundamental forces separated, and a slight matter–antimatter asymmetry left behind the matter we see today. About 380,000 years later the universe cooled enough for light to travel freely; that first light is still measurable as the cosmic microwave background, arriving from every direction.

The model rests on observation: the recession of galaxies (the Hubble–Lemaître relation), the measured abundances of light elements, and the remarkable uniformity of the background radiation. It does not say "why" the universe began, or what — if anything — preceded it; these remain open questions in physics and philosophy. The earliest knowable moment is bounded by the information the radiation carries to us.

The Big Bang is Eon's zero point: every event on this timeline unfolds within this expansion.