Why does the universe exist? Because God created it, answers the theist.
How did God create the universe? We have no idea. A common objection among some popularizers of atheism and on some internet discussion forums is that if we cannot know how God created the universe, then we are not justified in concluding that God did create the universe.
This claim is demonstrably false. Imagine the following fictitious discussion:
Julius Caesar: The sky looks so blue.
Brutus: How does the sky look blue?
Julius Caesar: I don't know.
Brutus: Then you can't say it looks blue.
Julius Caesar may not have had the advantage of accessing modern science, but it is plainly wrong to conclude that he wasn't justified in concluding the sky appears blue simply because he didn't know how its appearance is blue. One needn't know how something is true in order to know that it is true.