Tebor wrote:

I heard a theory that there's too much mass in the universe to be pulled back together and will keep expanding for eternity.

It depends on how much dark matter is in the universe. If there's a lot, our universe is "closed", and the universe will eventually stop expanding and then contract in the reverse of the big bang, leading to a cycle of "big bangs" and "big crunches" - which would also mean that time is cyclical. If there's not a lot of dark matter, the universe is "open" and will continue to expand forever. If there's a moderate level or "just right" amount, the universe is "flat" and will eventually stop expanding at a certain size, or the expansion will decrease to such a small amount that it will be inconsequential.

Scientists don't know how much dark matter there is yet, so whether our universe is closed, flat, or open is still unknown.