...any real proof as to why mankind believe in God or that there is no God.
#1- Not every one on this site, I even dare say, most every one on this site, is NOT an atheist.
#2- There is no such thing as "proof... that there is no god". There can only be proof FOR something. Ever. Period. That's how epistemology works. There can either be evidence for X or no evidence for X, there can never be evidence for "no X". For if X does not exist, then how could it's evidence exist either? You follow?
In this reality, there is no duality when it comes to extant evidence, like with matter and anti-matter. There is no such thing as anti
-evidence, to support a negative, and then evidence
to support a positive.
You could only ever prove that I HAVE two arms. You could never prove that I do NOT have a third(it might be hidden inside me, it might be invisible, etc.) You simply lack the belief that I have a third arm until evidence of a third arm is presented, just as it was presented for the existence of the other two.
In spite of what characters like Shockofgod or JP Holding might like for you to believe, the burden of proof is always on the proponent of a claim, never on the skeptic of a claim. History has taught us that if we want to hold as many true beliefs and as few false beliefs as we possibly can, then our safest path to that goal is to be skeptical of a claim until sufficient evidence is provided to justify belief in that claim, NOT to believe in all claims without prejudice until sufficient evidence is provided to abandon belief in a particular claim. Otherwise, we'd find ourselves believing in any irrational and dangerous bullshit that comes down the pipe.
Sorry to be cliche, but an atheist does not believe in a god for the same reason he/she does not believe in werewolves, fairies, or the flying spaghetti monster. The current amount of evidence for all of these things is insufficient to the atheist to justify him/her believing in them.
The burden of proof is not on you to prove that the flying spaghetti monster does not really exist. The burden of proof is on anyone claiming that it does, and until they provide sufficient evidence, your skepticism towards that claim is justified.
Even if a fact is actually true in reality, until sufficient evidence is provided, skepticism in that true fact is justified.
Even when everyone in the world believed that the earth was flat, if someone had come along claiming the earth was round, but had no evidence to prove that claim, then everyone would have been justified in not believing him, even though he ultimately was correct.
The ONLY thing that atheism entails is a LACK of believe in any god. It is not an organization or a dogma, it is just an adjective, and an adjective that only describes the sole attribute previously stated- lack of belief in a god.
Atheism is NOT...
...the claim that no gods exist, or could possibly exist, or ever did exist, etc...
...a belief that big bang is the origin of existence...
...the claim that there is no afterlife...
...the claim that there is nothing supernatural...
...the claim that the natural, material universe is all there is...
etc., etc., etc.
There are atheists who belief in bigfoot, aliens, ghosts, reincarnation, parallel universes, and all kinds of unconventional things.
And the burden of proof is on them to provide sufficient evidence for any of those things if they intend to convince anyone else.
For whatever reason, their personal standards of evidence have been sufficiently met for the existence of these things. Just not for a god.