If atheists believe that the big bang created the universe or whatever, then where did the big bang come from

Try the Physics or Astronomy & Space sections -- because no invisible magic man is required for this. But, since you asked here:

For thousands of years, people have said that their gods were behind what they didn't understand -- life, lightning, stars, earthquakes, the origin of life, the world or the universe, etc. Positing a god to supposedly answer a question solves nothing. It just adds an unwarranted level of complexity and stops you from asking more questions.

It used to be that science couldn't answer the question about the origin of the universe or of the Big Bang, but that didn't mean we should make up an answer (such as a god) and say that it was the cause. Within the last few decades science has discovered some good answers.

Quantum mechanics shows that "nothing," as a philosophical concept, does not exist. There is always a quantum field with random fluctuations.

There are many well-respected physicists, such as Stephen Hawking, Lawrence Krauss, Sean M. Carroll, Victor Stenger, Michio Kaku, Alan Guth, Alex Vilenkin, Robert A.J. Matthews, and Nobel laureate Frank Wilczek, who have created scientific models where the Big Bang and thus the entire universe could arise from nothing but a quantum vacuum fluctuation in the quantum field -- via natural processes.

In relativity, gravity is negative energy, and matter and photons are positive energy. Because negative and positive energy seem to be equal in absolute total value, our observable universe appears balanced to the sum of zero. Our universe could thus have come into existence without violating conservation of mass and energy — with the matter of the universe condensing out of the positive energy as the universe cooled, and gravity created from the negative energy. When energy condenses into matter, equal parts of matter and antimatter are created — which annihilate each other to form energy. However there is a slight imbalance to the process, which results in matter dominating over antimatter.