That we currently know of. This may sound strange to some. I believe existence has always existed.

I do not say that in a spiritual or metaphysical sense. My theory comes from the idea that for something to be created, either by nature or mankind, there has to be a substance from which that creation arises. Be it energy, carbon or whatever. Dark matter and dark energy are still mysteries even though we known it is there. We know it is there and it's called "dark" because we do not known exactly what it consists of.
I believe in the Big Bang but I think there had to be a combination interaction that was a catalyst to cause something the size of a pea to expand into the known universe. Multi-universes is a good theory but I think there is more to it. If there are multi-universes layered on top or all around ours then they are possibly being created all the time and have been created forever and always.

A mathematical concept cannot exist without zero. "Nothing from nothing is nothing." I known that is a song and a philosophical idea but I think the idea of "nothing" is incorrect if one believes that there is dark matter. I believe there is always "something". Even if it is defined as "zero". Zero is something. Minus zero and less is something.

Note that I am saying I "believe". Belief can lead to a theory, then scientific proof or not, but it can also lead to spiritual pie in the sky magic. Which does not exist.