There is an old saying 'A rolling stone gathers no moss', that is where the Rolling Stones got their name, and where Bob Dylan came up with the title of his song.
Dark energy is a hypothetical form of energy that is proposed by physicists to explain why the universe is not just expanding but is doing so at an accelerating rate. Some recent theories have suggested that it may be tied to the formation of black holes. But if it doesn't slow down galaxies will inevitability be pushed so far away from each other the night skill will be black, and then some time after that the expansion will be stronger than the force that holds atoms together and boom, the universe ends.
