If you have heat death, the remaining mass would fall back towards itself. These are all theories and the newer ones get replaced every few years. Dark energy is just a moniker to cover the fact that the math of the current expansion of the observable universe isnt mathing. We still dont actually understand everything, believe it or not.
These are all theories, but if the universe is a closed system then it should still follow the laws of thermodynamics (within reason). I dont think theres a point to discussing this stuff if youre gonna Google search stuff and esteem more value to current theories just because they are popular. Really most theories have value because thats how we learn. Its not a popularity contest.
It all returns to nothing
It just keeps tumbling down, tumbling down, tumbling down. (NGE)