The big bang is the moment that space and time (or ‘spacetime’) came into existence. Before the Big Bang there was no space or time. So, it is actually meaningless to ask what caused the Big Bang to happen – there was no Universe in which that cause could have existed. This might seem like a bit of a cheat, but there are other good reasons to suppose a cause for the Big Bang might not exist. Quantum physics has shown us that some events have no cause at all. Things can happen randomly, spontaneously, and for no particular reason. This unpredictable and ‘causeless’ nature of the Universe is experimentally verified but has nothing to do with our inability to observe correctly – it is a fundamental property of the Universe. So, although there may have been a cause for the Big Bang that we are unaware of, modern cosmology neither defines nor requires one.