As I walk through a large expanse of trees, I am struck by the fact that I cannot see the forest, only part of it at a time, yet I still see the forest.

I cannot step back and see the whole of the universe, but I see what is around me. That is evidence prima facie that the universe exists. We may not know the extent of it, or many othre things about it, but its existence in front of my face is enough to settle its existence for me.

Or were you infering that it only exists in your mind?