The Big Bang occurred approximately 13.8 billion years ago. This is the widely accepted estimate based on detailed measurements of the universe's expansion and observations such as the cosmic microwave background radiation

. Modern cosmological data, including from missions like WMAP and Planck, support an age close to 13.77 to 13.8 billion years, with a very small uncertainty margin

. While some recent studies have proposed alternative, much older ages (e.g., around 26.7 billion years), these are not yet widely accepted or confirmed by the broader scientific community

. The consensus remains that the universe began with the Big Bang roughly 13.8 billion years ago, marking the start of space, time, and matter as we know them

. In summary:

• The Big Bang happened about 13.8 billion years ago.
• This estimate comes from measuring the universe's expansion and cosmic background radiation.
• The age is supported by multiple independent observations and cosmological models.
• Alternative older age claims exist but are not mainstream.

Thus, the Big Bang is considered to have occurred roughly 13.8 billion years ago.