You know this ... because? You wished real hard?obviously , the Universe exists ; but never existed before ..
there had to be a beginning .
You read it on the back of a cerial box?
You heard it from a scientifically illiterate moron raving from a pulpit?
In any case-- you do **not** know that the universe never existed before.
Nor do you know it had a beginning-- the present universe may well have existed in some other form, prior to it's current state.
And you have no way at all to know otherwise.