There’s an old theory of creation that tried to reconcile a belief in long ages (billions of years vs. thousands). There are many problems with this theory, ultimately relegating it to the dustbin of history. But, it unfortunately still crops up from time to time, so I’ll offer my thoughts on it. There were variants, but the main ones put a time gap between Genesis 1:1 and Genesis 1:2:
In the beginning God created the heavens and the earth. (Genesis 1:1)
The earth was formless and void, and darkness was over the surface of the deep, and the Spirit of God was moving over the surface of the waters. (Genesis 1:2)
The biggest problem, in my opinion, is that it doesn’t solve the problem it was created to address. The problem is that secular science holds that the universe and everything in it happened gradually over millions of years. The gap theory postulated that there was a whole creation before verse two, and something happened to it, causing God to recreate everything. This second creation happened at the same time as young-earth creationists put it, just a few thousand years ago.
But, the long ages hold the fossil record. The fossil record is ostensibly where the long ages come from in the first place. So, the gap theory solves nothing that the biblical creation model doesn’t already solve. So, the Gap Theory should be dead on that account.
But, there is yet another fatal flaw in the concept: the universe. The billions of years idea applies to stars and galaxies as well. Yet scripture is very clear that the sun, moon, and stars were all created on the fourth day, after the gap.
I recommend that if you hear someone speaking favorably of this theory, point them here, or to Creation Ministries International, which has an large collection of articles on this idea.