Christians, how do you explain the existence of God?

Christians always say that Atheists have a problem because they can't answer the question "Why is there something instead of nothing" and That they don't have that problem because they believe God created the something. But then I can't help buy wonder Why God exists? If you reduce the something back from the universe to God then you're just left with having to explain God, so I ask you theists, How do you explain God?