Hi sgrover! The Big Bang happened 13.75 billion years ago! We have such a precision measurement for many reasons. The best evidence we have for the Big Bang is something we call the cosmic microwave background (CMB) radiation. Its a bath of photons (particles of light) with a temperature of around 2.7 kelvin (that’s pretty cold!). These photons have energies in the microwave region of the electromagnetic spectrum. The universe is literally bathed in these relics from the Big Bang! They can even contribute to the static on old tv sets – imagine Big Bang static on you tv!!! Early on in the history of the universe the CMB photons were much much more energetic as the universe used to be a lot hotter! Especially as we get closer and closer to the Big Bang. As the universe has expanded it has cooled and the CMB has cooled with it. We use some clever physics and the CMB’s temperature to trace back when in time the Big Bang occurred! Pretty cool!!