We weren't there to see it; but I'll bet "there was light!"
The earliest epoch of the universe which we can observe, inflation, was completely empty except for the field that drove inflation. There was no light, no other matter of any kind. Just this one, very high-energy field. When that field decayed, then came an extremely hot soup, so hot that today we don't yet understand physics well enough to say precisely
what it was. But what we can say is that it was full of all sorts of particles, including photons (light).
But regardless of these rather small inaccuracies of the statement early in Genesis, "Let there be light," what precedes that statement makes
no sense whatsoever in the early universe.
Consider what it says in Genesis 1:
" 1In the beginning God created the heaven and the earth. 2And the earth was without form, and void; and darkness was upon the face of the deep. And the Spirit of God moved upon the face of the waters.
3And God said, Let there be light: and there was light."
Note the first statement: God created the heaven and the earth.
This makes no sense! Even though you
might call the post-inflation hot soup "light" in some sense, there was nothing at all that might be called the earth at that time! It simply makes no sense! All that existed within our region of the universe was this obscenely smooth, uniform field that drove inflation.
The words "face of the deep" and "face of the waters" also make no sense whatsoever at this epoch of the universe: there was no water, or any fluid that was anything like water, and there was no boundary that might be called a "face".
Finally, there was no "let there be" going on: the "light" came from a fully naturalistic process, the decay of the inflaton field. To say "let there be light" here would be like timing a stop light and saying "Turn green!" at just the right time, and expect your friends to be amazed when you get the timing right.
Things only get worse when you try to shoehorn the rest of this primitive creation myth into science.