As human beings, we are of the flesh, which has its boundaries. We understand the span of time and are limited within the context of the world in which we live.
We are not able to comprehend eternity; the idea that there is no beginning nor end of existence. How can that be? Everything must have a birth and death. But, in the spiritual realm, time is not relevant. There is no yesterday, today, or tomorrow; but instead existence.
GOD does exist within the bounds of time, or even space as we know it. And in our limited ability to ever understand how GOD "is," we often make the mistake of concluding that GOD does not exist; the ignorance of 'mortalness.'
Eternity can be a fearful concept, especially when nobody knows the answer. In the novel, Day of Rain, I made an effort to expound upon the complexity of omnipresence; being in all place at once. Quantum Physics only touches the surface of this question, but none-the-less, does help with some validation of how this can be.