How did our universe come into existence?
Although there is not even a theory of what set off the "big bang," it is the starting point of our universe. Astro-physicists theorize that the entire universe was compressed into what is called a "singularity" just prior to the big bang. A singularity is where Space-time ceases to exist, that there is only 1 dimension and not the 3 we live in.
No one knows, the present universe is thought to have been caused by an explosion of the singularity called the big bang.
The big bang explains where our present universe came from but it doesn't where the singularity came from.
For years the theory was that the universe was eternal and that the universe oscillated between Big Bangs and Big Crushes. The idea was that the mass density of the universe was bigger than omega and that the expansion of the universe would slow down resulting in the original singularity being reformed.
In 1998 it was discovered that the rate of expansion of the universe was speeding up not slowing down as hypothesized. This means that the universe has a density of less than one Omega and the universe will not come back together to reform the singularity.
The universe is empirically found to be an open system which indicates that there is something that exists besides our present universe. The present universe had a beginning the Big Bang but it is going to expand infinitely never recycling.
The present universe began with the big bang but what caused the matter and energy to exist that caused the big bang is a mystery.