How can we understand the World in which we find ourselves? How does the universe behave? What is the nature of reality? Where did all this come from? Traditionally these were questions for philosophy, but philosophy is dead, philosophy has not kept up with modern developments in science, particularly physics.
philosophy of cosmology is a new field of study within the philosophy of physics which aims to bring a philosophical approach to the basic questions of physics, including those concerning the nature, age and fate of the universe. Some people simply proclaim that God created the universe. Apart from, there is very good evidence that the universe did come into existence in a Big Bang, about fifteen billion years ago.
So we are faced with the problem of what happened beforehand the Big Bang. If the Big Bang was the beginning of time itself, then any discussion about what happened before the Big Bang, or what caused it seems simply meaningless. The Big Bang was very much a sudden, explosive origin of space, time, and matter.
First, time did not always exist; and second, there was no first moment of time. Science is good at telling us how, but not so good on the why. Maybe there is not a why. To wonder why is very Human, but perhaps there is no answer in Human terms to such deep questions of existence. Or perhaps there is, but we are looking at the problem in the wrong way.
What happened before the Big Bang?
The answer is, Nothing.