Two of the hardest science questions are, what is the relationship of gravity to the other basic forces of the universe, and what is the dark energy which apparently is causing the expansion of the universe to accelerate.
Diamond (moncrystaline carbon) is the hardest mineral on Earth at least. The universe is quite varied and we know very little of the hardness of neutron stars or black holes...
In the Universe the largest object structurably is Sloan Great Wall which is 1,370,000,000 light years wide and the biggest physical object is the Observable Universe Which is 93,000,000,000 lights years in diameter
Every object in the universe is part of the universe because everything such as air,desk,chairs, anything is made up of atom which the world is made up of one huge atom.