Why disbelievers believe? They believe in so called “God of the gaps”.

[FatAndFaithless]

"God of the gaps" is a term used to describe observations of theological perspectives in which gaps in scientific knowledge are taken to be evidence or proof of God's existence... how wrong it is to use God as a stop-gap for the incompleteness of our knowledge. (Wikipedia 2016).

Theorem: it is wiser to let the God in the ship's hold gap, until the secular science will find the secular stop-gap. Proof: "it is just obvious." More is here:

But while the "white spot" or the “gap” (the gap in the secular view of the world) is not closed by the secular science, let the gap be closed by the True God Himself. If is needed, the God will make the additional room for the secular science. But why to tear the God from all the white spots? Otherwise our society (like a ship) will sink in the "sin-deeds-sea" due to huge ugly holes in the knowledge!

There is nothing wrong with a gap in our knowledge, and I think it is better to say I (we) don't know.  And it's not just theist who do it, but a gap epiestomology is quickly uncovered by asking the simple question of why?

You're right. There's nothing wrong with gaps in knowledge, and it's both more honest and more responsible to say "I don't know" when confronted with one of these gaps.  But that's not what happens with many theists.  You present a gap in our knowledge - the origins of the universe for example - and instead of saying "I don't know" the response you receive is "God is the reason."