This is a rather weak argument known as the "God of the Gaps", that anything for which science does not currently have an explanation must be the handiwork of God. (This has the unfortunate side-effect that any time scientific knowledge increases, God's domain is diminished.) It's much more honorable to simply say "I don't know".
If science points to a designer, then it is not "God of the Gaps." In other words, we all know how to detect design, but naturalists refuse to apply this everyday ability to their own theory. So most of the time their claims of "God of the Gaps" are invalid. On the other hand, I have seen them countless times invoke "Science of the Gaps" such as "we don't have evidence now, but someday we will because we have already decided a priori that naturalism is true."