> Is there a difference between 'belief' and 'knowledge'?
Yes. Knowledge cannot be wrong, beliefs can. A consequence of this is that true knowledge doesn't actually exist, but beliefs can become sufficiently reliable that calling them knowledge is a pretty reasonable approximation.
> Why does the universe exist?
We don't know.
> why is there something rather than nothing
That is a different question. We actually do know the answer to that: it's because "nothing" is unstable.
Is there a difference between 'belief' and 'knowledge'?
Why does the universe exist? (This is, I think, the core of 'why is there something rather than nothing?')