The Big Bang is not an explosion in space. It is an expansion of space itself. About 13.8 billion years ago, the universe was hotter and denser than anything we can reproduce. It has been expanding and cooling ever since. That expansion carries galaxies apart, stretches light into longer wavelengths, and leaves behind a faint glow of microwave radiation.

Three observations support the model. First, the universe is expanding, as Edwin Hubble showed in 1929 by measuring galaxy redshifts. Second, the cosmic microwave background fills the sky with radiation at 2.7 Kelvin, exactly what a hot early universe would leave behind. Third, the abundances of hydrogen, helium, and lithium match predictions from the first few minutes after the Big Bang.

The model does not explain what came before or what triggered the expansion. It describes the universe from a fraction of a second onward. Inflation theory proposes a brief period of exponential expansion in the first instant, which would explain why the universe looks so uniform today.

The name is misleading. There was no bang. There was a beginning, and we are still living in its aftermath.