The hypothetical but widely accepted ‘explosive’ event that marked the origin of the universe as we know it. At the time of the Big Bang, the entire universe was squeezed into an infinitely small, hot, superdense state. The Big Bang explosion threw this compact material outwards, producing the expanding universe seen today (see redshift). The cause of the Big Bang is unknown; observations of the current rate of expansion of the universe suggest that it took place about 13–14 billion years ago. The Big Bang theory began modern cosmology.

According to a modified version of the Big Bang theory, called the inflationary theory, the universe underwent a rapid period of expansion shortly after the Big Bang, which accounts for its current large size and uniform nature. The inflationary theory is supported by the most recent observations of cosmic background radiation.