I've been thinking about this some more. The biblical account of"Let theire be light" aligns with the current secular theory of the photon phase of the Big Bang. Separation of light from darkness requires more than just objects to block the light. It requires the light to only come from some locations, not all locations.Thus, we can understand that, conversely, if there is no separation between light and darkness, that light must have been everywhere, filling the universe, where there was no shadow, no darkness, because darkness disappears when light is present.
If the locations of light were far enough away from each other, it would work similarly to what we see now, but without distinct bodies of light. And the earth, spinning close to one of those bodies (not yet localized enough to be the Sun) would experience evening and morning periods before the sun became a distinct entity (remember, there were photons and light before the sun).A thought just occurred to me. What if this meant that photons originally could have passed through matter, and the "separation" would be God making it so that light couldn't pass through matter, "dividing" it like placing a rock in front of a flashlight?
You can see that something more is needed, because the shadow of the earth ("evening") wouldn't happen if the light still came from everywhere.The universe already existed, all God would have to do is make it so that objects created shadows, and they would be what "divides" the light, separating light from darkness.
You might remember a song by Don Francisco, "He's Alive", in which Jesus first appears to Peter after the resurrection, with the line "Light that came from everywhere drove shadows from the room."