Is universe expanding or contracting?

As its a fact after big bang our universe started to expand. But the question is upto what extent it would expand?

There might be some limit. Then may be after that it would start to contract towards it centre. This can be the end of universe! If there is a beginning then there must be an end of it.

This can also be the reason why we are seeing speedier separation of galaxies. Galaxies closer toward centre accelerating speedily towards centre.

After big bang a force was exerted in outer direction and universe is said to expand. But then an equal force must have also been exerted in inward direction.
So the centre started to shrink down and became a great central mass or may be a universal black hole. It is attracting attracting other galaxies.

There arises also a question why big bang occurred?
This all started from a zero after big bang, then what made this zero to bang and expand?
This question we all might think of many times but end up stressing myself with nothing.