Is our sun the centre of the cosmos ?

Apparently yes.

That is because all places are at the center of the cosmos./ universe

Confusing ?

The big bang expansion is the same everywhere.

So we see galaxies in all directions receding from us. And at the furthest distance we can see, the galaxies are receding at almost the speed of light.

But no matter where you go in the universe, your view is the same.

Hence all places are at the center.

It follows from that, that it is impossible to leave the universe.

Funny you ask, you should read the hilarious novel by Douglas Adams called "Hitchhikers Guide to the Galaxy". In it, Adams talks of a torture device that is designed to show people the utter insignificance in the grand scheme of things and, therefore, the futility of their existence -- until it didn't work on one particular individual because, it turned out, the universe actually did revolve around him.

P.S. This is not a serious answer.

Yes.

Every point in the universe is the center of the cosmos.

No it is just the center of our solar system. The "cosmos" has no center.

our sun is a nothing in the universe

no

No.