What is usefulness of such a vast universe!!

That's not really a meaningful way to think about it. However, two things i suppose: the mind of God is unknowable and beyond us, and that a work of art can be great where "less is more", for instance Paul Klee's Black Square. Both of those, obviously, assume there is a God of some kind.

That's not a proper way to think over it. You must think that there are lots of matters in this universe to study/think over & u should try to explore it through scientific manner.
Just consider the universe as the study material.

none......if u're considering this question, why not consider this hypothetical question: what is life's purpose here in this universe?

It is vast but finite,it Simply conforms to the circumstances that arose from it's emergence.
It is likely no more than 6 billion light years in radius.

May I suggest that usefulness is not a requirement for existence?

The universe was not designed for us and has no purpose, it just exists.

No use at all.