That's a simple one: Because time itself began with the Big Bang, there was nothing before it. 
Ehm, creation out of nothing? It doesn't make a sense for me, this is not sufficient answer, my mind is to narrow and wants for every question some logical answer 
- but at the same time Because time itself began with the Big Bang, there was nothing before it. is the only theory I can accept cause there's no better available theory at this moment.
Sorry I know, that now I look stupid, but that's me, chronic doubter 
I understand you very well. 
"Because time itself began..." is only right within the current standard theory of the Big Bang. If the theory proves all wrong or needs to be modified, then the answer will have to be different. This is science: Don't hope for ultimate answers - these are the domain of faith - but always be ready to doubt, and always be aware that every answer you get is implicitely "As far as we know today..."
The trouble with these questions regarding the whole universe is that they are inaccessible to human imagination. How many universes have we seen being created? Then how can we judge if creatio ex nihilo or eternal existence is more absurd? Our imagination is trained for earthly scales: Centimeters to kilometers, seconds to years, a few hundred Kelvin... what is far below or far above these is unimaginable to us and only accessible to rational thinking, i.e. math. That's why there's no point in waiting for an explanation that sounds understandable.
Well in light of previous discussion...
The closest theory to me, is that one, saying that space as we know it now, was created from chaos... it sounds acceptable that there was just something like huge mess called chaos and in this chaos
accidentaly had appeared a germ of order...
Becuase when I admit that there was some order arose not by coincidence, it automaticaly aims to question: What (how was/ or by who...) created the order? (And that also aims to: How was this "Creator"/or whatever You want created, etc...)
Hmm... and I can ask from where came the chaos as well...
These questions are like chain - You can't see beginning nor the end - It reminds me of some kind of snake 
