Simple answer: nothing.
When you search for the source of a stream and find it, you don't have to look any further. When you find the highest point on a mountain, you don't have to climb any higher.
When an artist paints a painting, you don't need to ask who painted the artist.
Space, time, energy and matter are artificial constructs. They possess continuity and the other three within time possess persistence. That which would create such as these could not be made of these. The creative source of space would not take up space, it would be timeless, not require or contain energy, and certainly not possess mass. Each of these "things" can be acted upon -- they are at "effect." The source of these is at "cause." In the literal sense, there was no "before creation" or "outside of creation." These are mortal concepts of the creatures held within that created universe.
I have witnessed the creation of "extraordinary miracles" (the kind which circumvent the laws of physical reality. Some of these miracles were performed repeatedly. Each time, the result was instantaneous. I have also witnessed being outside of this physical shell (Homo sapiens body) with full visual acuity. Each of these events suggests that the true self, within, is not a product of creation, but a source of creation -- an immortal child of God.
Genesis describes "man" as created in the image of God; i.e. baby gods. For anyone to walk on water, part the sea or any of the dozens of miracles I've witnessed, they have to be not of this physical realm, but of the realm of creation (timeless, spaceless, without physicality). And Genesis does say let "us" make man in "our" image (plural). This is the spiritual part of man and, at one time, one with God -- currently a blazing amnesiac who typically thinks they are their ego and body.
(After 60 years, I'm still working on this answer. If you want a better answer, try me in another 60 years.)