Say God doesn't exist...?

As a theist, I do not understand how someone doesn't believe in God. So, I am asking those who don't believe in God for their proof that He doesn't exist. How do you know there isn't a God, and what explains everything on this earth? How did humans arrive here?
Update: This really isn't a yes, no, maybe question. Explain to me in details the point of not believing in a higher purpose.
Update 2: I am not an atheist, and I am not making any attacks on atheists. I am simply just trying to understand what they believe in.
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