What is the gap theory ?
The main idea behind the gap theory is, that lucifer rebelled against God and persuaded 1/3 of all the angels to rebell with him, and in attacking the throne of God, he was thrown out from Gods presence, and stripped of his authority he had with God.
Genesis gives some clues as to the situation in saying that the earth was tohu wa bohu, in Isaiah God says that He did not create the earth like that, but created it to be inhabited, in other words, the earth was not tohu wa bohu in the beginning, but it became like that, which the septuaginta also states.
Some claim that the gap theory is relatively new, that Thomas Chalmers made it up to give in to that science concluded that the earth was old, but the gap theory can actually be traced back to the time of Jesus, or, as I would mean, to the time of the translating the hebrew of genesis to greek, because it uses different words than the modern translations do.
The gap theory I claim, is as old as the bible itself, and not just a new idea.
The good thing about the gap theory is, that is gives answers to questions the other views dont deal with.