The Big Bang occurred approximately 13.8 billion years ago. This estimate comes from measurements of the universe's expansion rate and observations such as the cosmic microwave background radiation, which together provide a consistent timeline for the age of the universe

. In more detail:

• Astronomers use the rate at which galaxies are moving away from each other (the Hubble constant) to extrapolate backward to the moment when the universe was concentrated in a hot, dense state, marking the Big Bang event about 13.8 billion years ago

• This figure is supported by multiple independent methods, including observations of the oldest stars and the cosmic microwave background radiation, which is the leftover heat from the early universe

• While the exact cause of the Big Bang remains unknown, the timing is well established and widely accepted in the scientific community

Thus, the universe began with the Big Bang roughly 13.8 billion years ago.