Question

since the ------------the universe has expanded and cooled greatly​

Answer 1

since the whole universe had expanded and cooled .

Answer 2

Answer:

Since the big bang theory the universe has expanded and cooled greatly​

Explanation:

This is a theory that tells us that how the universe developed. When universe expands it also cooled and converts the energy into subatomic particle such as electron, proton and neutron. Before big bang combing of neutron and proton was taking years but after this big bang it takes only minute about few to form nuclei. The universe is finite and it has no end but if the universe was bound it will expand forever.