"The prevailing model for the evolution of the Universe is the Big Bang theory. The Big Bang model states that the earliest state of the Universe was an extremely hot and dense one, and that the Universe subsequently expanded and cooled. "
"In a universe that was essentially static, there would not have been any dynamical reason, why the stars should have suddenly turned on, at some time. Any such "lighting up time" would have to be imposed by an intervention from outside the universe. The situation was different, however, when it was realised that the universe is not static, but expanding. Galaxies are moving steadily apart from each other. This means that they were closer together in the past. One can plot the separation of two galaxies, as a function of time. If there were no acceleration due to gravity, the graph would be a straight line. It would go down to zero separation, about twenty billion years ago. One would expect gravity, to cause the galaxies to accelerate towards each other. This will mean that the graph of the separation of two galaxies will bend downwards, below the straight line. So the time of zero separation, would have been less than twenty billion years ago. "
I think that the universe has always existed and it is in a fluctuating state.
I am not a scientist, but I just can't take either theory - static or big bang. What we consider expansion is just a too local and short-time observation.
Philosophically it is also fruitless to think about WHY the universe exists (if it exists in the first place). The nature and universe, including life and evolution on the planet Earth, don't have any reason intrinsically, no agenda or purpose. It just exists.
PLEASE, keep this discussion polite and free of religious proclamations. If your explanation includes the creator (a god of some kind), that is okay to tell.