Atheism is the belief there is no God. Most self professed atheists are better labeled as agnostic meaning they aren’t sure.

There are only three possible explanations for the existence of the universe (Full Article):

1.  The universe created itself from nothing. Illogical (0+0=0)

2.  The universe has always existed. Violates the laws of thermodynamics. How did the original mass/energy come into existence?

3.  The universe was created (God). Who created God? "God is a spirit:" - John 4:24. He is 
outside of the universe (time, space and matter).

“For the invisible things of him from the creation of the world are clearly seen, being understood by the things that are made, even his eternal power and Godhead; so that they are without excuse:” - Romans 1:20 KJV