The question seems reasonable enough, but it overlooks the eternality, transcendence, and self-existence of God.
I like this turn of the tables: "If man created God, then who created man?"
Since human beings are clearly not eternal, transcendent, or self-existent, and the person asking the question is a human being, this seems to be the more important of the two questions. Skeptics say they cannot see or sense God, so how can they know He exists? Okay, fine. But all of us can perceive human beings, and must somehow account for their existence. As I see it, the choices ultimately boil down to spontaneous generation in an illogically eternal universe, or creation by God (which may sometimes appear illogical to some, but is ultimately the most logical of all possibilities).
Someone will say that skeptics don't know the eternality, transcendence, and self-existence of God, which is why they're asking the question in the first place. The Bible says they most certainly do know these things, and they are therefore without excuse in their stubborn refusal to glorify their Creator by trusting in Him.
Romans 1:18-21 For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men who suppress the truth in unrighteousness, because that which is known about God is evident within them; for God made it evident to them. For since the creation of the world His invisible attributes, His eternal power and divine nature, have been clearly seen, being understood through what has been made, so that they are without excuse. For even though they knew God, they did not honor Him as God or give thanks, but they became futile in their speculations, and their foolish heart was darkened.
It is not a lack of logical explanation, but a lack of trustful adoration, which prevents sinful man from believing in his Creator.
God exists as the uncaused Cause, the unsourced Source, the unmoved Mover, the uncreated Creator, the unmade Maker, the ungiven Giver - and all too often as the unloved Lover. Even those who deny this know it to be true.