We know the universe started with a 'big bang' because as Professor Brian Cox puts it, "we can see it"

If we accept that then as others have said the question arises as to what caused the 'big bang'.  The answer at the moment is, "nobody knows".  Stephen Hawking theorises that there was nothing prior to the 'big bang' - most importantly no time.  His explanation of the singularity that started the universe is quite frankly way over my head, linked to M-theory and the instantaneous creation of multiple universes.

Hawking also suggests that because time was created by the 'big bang' then the idea of repeating 'big bangs' is not possible.

There are patently forces in the universe that we haven't yet discovered - there must be something that is causing the universe to expand at an increasing rate, this has been named 'dark energy' within the scientific community but has not yet been directly detected.

All this begs the question, "how do I think the Universe began?" - the only answer has to be, "I don't know" - how can I make a suggestion when I can't even understand the suggestions put forward by the leading minds in this field.  I might just as well suggest it started with a giant jam donut!