@Akio
I wonder what you mean by "the behind"...
As far as we can tell, everything that came from this Big Bang is 13.8 billion years old (with the mandatory margin of error). This includes the observable universe, but also matter that lies beyond this boundary, because of how the universe expanded at great speeds (especially in the beginning).
But then there's also the possibility that this isn't the first Big Bang event, and the Universe keeps creating matter through these events, expands and spreads it until "heat death", and then repeats itself.
And then there's also the always popular theory of the Multiverse, with countless Universes that go through these processes next to each other.