Our universe and our existence would be the same as the Game of Live. Nothing need exist except the rules of math. You don't ask what comes below the bottom of a parabola, the same with our universe. The start is just where the rules start from a singularity. There is nothing before it because time is just a parameter that has no meaning before the singularity. Just has -1 y means nothing to the parabola y = x^2. The start of the parabola universe is at x=0 and there is nothing before it. However the Parabola Universe is not complex enough to contain sentient creatures such as ourselves. But there are infinitely more definable universe all with real existence in a sense, but then again only those complex enough to contain thinking creatures might be called/perceived as real. Given the infinite universes that then exist, there would indeed be some running simulations that create simulations of our universe, but our existence doesn't depend on those simulations being run, it merely gives those universes a window into ours.
I had started on a few occasion to put pen to paper to write these ideas down, but it appears I was beaten to the punch by Max Tegmark and his Mathematical universe hypothesis"