I belive this is no flying pink elephent behind me, but I can't look back.
Is it realy a belief?
No, there's a difference between God and a physical object.
It's impossible to prove that something doesn't exist.
Yet, some people say there cannot be a God because there's no proof of existence. But, they when I ask the same question, they can't give me an answer.
Yes, some atheists make a positive assertion that it is impossible that God exists. Most do not. Saying that something does not exist is not a positive assertion. Saying that it is impossible for something to exist is.
A positive assertion postulates something. Saying that something exists is a positive assertion. A negative assertion merely negates a positive assertion. Saying that something does not exist is a negative assertion. Saying that it would be impossible for something to exist is a positive assertion.
This subtle difference is important because the standards of evidence are different. A negative assertion in effect says that "there is no reason to believe [insert positive assertion here]," while its positive counterpart says that something is impossible.
As a side note, saying that it is not impossible for God to exist is also a negative assertion. In fact, I think that the vast majority of atheists would agree with that statement.
Saying "There is no God" is a positive statement. Look at it. It doesn't say There can't be a God, which is negative. IT says that their belief is that there is no God, hence a positive assertion of that.