The following text is based on current scientific assumptions,
which are highly speculative.
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.
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.)
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.