The Masoretic Hebrew text of Genesis 1 indicates a break at verses 1-2. Those ancient Jewish scholars incorporated little indicators into the text to aid the reader in pronunciation and interpretation. It is significant that they inserted a small mark (called a "Rebhia") after verses 1-2. It serves to inform the reader that there is a break at that point in the narrative. These Rebhias are not in the original manuscripts, and therefore, only represent the considered judgment of the Masoretics, but their scholarly judgment ought to have some weight in this matter.
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In summary, these evidences surely indicate a gap between God's two works of creating and making in Genesis 1. We are, therefore, persuaded that the gap is not a "theory," but is something that has Scriptural support.
There May Be Two Gaps!
Walter Scott, E. Schuyler English, and others, suggest that there very well may have been two gaps! They are not dogmatic about this, but they suggest it to show that we really cannot go beyond what is stated in Scripture. The first verse states that God created the heavens and the earth; we are not told how long it stood in that pristine state: Hence, the first gap. Then, we are told that the earth was laid waste by some upheaval or judgment. Again, we are not told how long the earth laid in that state; hence another undisclosed period—a second gap.