There are a few different ways that atheists explain the existence of something from nothing, or the Big Bang.

One way is to simply say that we don’t know how it happened. The Big Bang theory is a scientific theory, which means that it is based on evidence and observation, but it is not a complete explanation of everything that happened. There are still many things that we don’t know about the Big Bang, and one of those things is how something could come from nothing.

Another way that atheists explain the existence of something from nothing is to say that it is possible that the laws of physics were different at the time of the Big Bang. We know that the laws of physics have changed over time, so it is possible that they were different at the very beginning of the universe. If the laws of physics were different, then it may have been possible for something to come from nothing.

Finally, some atheists believe that the Big Bang was caused by a quantum fluctuation. Quantum fluctuations are tiny, random events that can happen in the vacuum of space. These fluctuations can create particles and energy out of nothing, so it is possible that a quantum fluctuation caused the Big Bang.

It is important to note that these are just a few of the ways that atheists explain the existence of something from nothing. There is no one definitive answer to this question, and it is a topic that is still being debated by scientists and philosophers.