Why don't atheists believe in any gods?

Original question:

Why don't atheists believe in a God?

From what I know, atheists believe that there was particles floating in nothing, that suddenly came together to create something. How could the set amount of particles in that area create space that is infinite? How isn't there a God? It all has to start somewhere..

To answer your question: because if they believed in a god they wouldn't be atheists to begin with.
OK, what are the reasons of our disbelief? Well, you know, atheism is not a group, atheism is just a shorthand for People who doesn't believe in gods or supernatural creatures associated with gods nor follow concepts, laws and rituals associated with said gods. That means that it is impossible to answer for all the atheists, because each individual atheists has his or her own reasons for not believing in a god. In my particular case, because one fine day, while at mass at school (I went to a Catholic school), I realized that all the ceremony I was experiencing wasn't any different from the ceremonies of the alledgedlly "false" or "heathen" religions, so if the other religions were false and their gods didn't exist, then this god doesn't existed too and the Catholic religion was as false as the rest. OK, I know that my reasoning is flawed but I was 10 back then. But it was the first step in a search of information about gods and religions and after reading a lot about mythology, history of religions, archeology, anthropology, sociology, psychology and other things, plus several of the holy books I am 100% sure that all the know gods are human inventions. About the unknown gods I am not so sure, but since we don't know anything about them (they are unknown, duh) nothing can be said about them, nothing can be done or undone to please them and, technically, it is the same if they exist as if they don't exist, so I don't believe in them either.

Now, about the question details...:
From what I know, atheists believe that there was particles floating in nothing
Well, don't be offended when I tell you that you know nothing, Jon Snow.
As it is said above, atheism is not a group so there is no common belief about anything. So you have been misinformed, because atheists don't believe that there were particles floating in nothing.
Anyway, there were, are and will be particles floating in nothing. Everything (including you and me) are technically particles floating in nothing. Yes, I know, everything looks so solid and material but the truth is that all the atoms that constitute matter are esentially a lot of "nothing" with some particles "floating" in it.

, that suddenly came together to create something.
Anyway, particles are already something, so actually the particles floating in nothing didn't came together to create something because they were something, but yep, that's what electrically charged particles do, specially when the atomic orbitals have still room for more electrons.
It is better explained in Physics and Chemistry books, but atoms tend to bond to other atoms and form molecules, which are a bigger kind of something. And molecules tend to bond to other molecules and form matter, which is a much bigger kind of something.

How could the set amount of particles in that area create space that is infinite?
Obviously you didn't read what you wrote. How can exist an area before space was created if an area is a portion of space?
Space and time were created after the Big Bang, together with matter. Before the Big Bang... there was no before, since there were no time.
Yes, it is difficult to understand, because things happening at the begining of time and space are counterintuitive, and there is still a lot to know about.
Anyway, this is something an astrophysic can answer, not an atheist. I know, I know, the two words look so similar so it is easy to mistake one for the other, but, believe me, it is an astrophysic and not an atheist the person who can give you a proper answer to this question.

How isn't there a God?
Why should it be? So far, for all we know everything happened according to some simple characteristics of charged particles floating in nothing.
And anyway, why "a god" and not "a human being" or "an alien"?
Maybe our whole universe is the effect caused by an experiment yet to be done "at the end of times" in our universe. We are in paradox territory here. A mystery doughnut...
What I mean: the "God-did-it" explanation is a non-explanation, the peak of intellectual lazyness, plain ignorance. "Something magical happened and poof our universe appeared".

It all has to start somewhere..
Of course. It all started with a Big Bang.

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