Atheists do not believe in the existence of a god or gods, so they do not believe that the universe was created by a god. Instead, they believe that the universe came into existence through natural processes. There are many different theories about how the universe came into existence, but there is no scientific consensus on any one theory. Some of the most popular theories include:

* The Big Bang theory: This theory states that the universe began as a very hot, dense point about 13.8 billion years ago. It then expanded and cooled, forming the stars, galaxies, and planets that we see today.

* The oscillating universe theory: This theory states that the universe has been expanding and contracting forever. It has gone through many cycles of expansion and contraction, and will continue to do so forever.

* The multiverse theory: This theory states that there are many universes, each with its own laws of physics. Our universe is just one of many universes in a multiverse.

It is important to note that these are just theories. There is no scientific evidence to prove any of them. However, they are all based on our current understanding of the universe, and they provide possible explanations for how the universe came into existence.

It is also important to note that not all atheists agree on the origin of the universe. Some atheists believe that the universe may have always existed, while others believe that it may have come into existence through natural processes that we do not yet understand.