The following text is based on current scientific assumptions, which are highly speculative.
(see http://timelines.ws/)


Before the Big Bang
In a vacuum state with no space or time, physical laws would not seem appropriate. However, the law that states matter can neither be created nor destroyed implies another state here, that is: a state of pure energy unbound by space and time. The chance fluctuation indicated below for the beginning of the Big Bang would have occurred in this energy field. This occurrence could have been like the breaking of a dam, or a bomb that violently explodes upon detonation. The resulting tiny bubble of space-time provided an outlet for the enormous energy latent in the pre-space-time state.

(This of course gives no account of how or where or why the initial pure-energy state came about. We may never know, but we can always speculate.)


The BIG BANG
While in a vacuum state a chance fluctuation occurred in the void owing to quantum uncertainty. This yielded a tiny bubble of space-time. Before it could revert back to nothing, a sort of negative pressure caused a runaway expansion faster than the speed of light. Time, space, matter and energy came into being. Matter and energy began to define space and time.
(This is described in the 1997 book: 'The Inflationary Universe' by Alan H. Guth.)

Estimates of the age of the universe are in the range of 20-8 billion years, but most astronomers believe it all began about 15 billion years ago.