Fly around the universe collecting matter then find or create a black hole of appropriate size and farm the gamma rays, small ones generate quite a lot of power and you can keep them at that size by feeding them. Humanity won't run out of energy for at least 10^100 years. Theoretical physicists suspect that protons have a half life of 10^32 years, that's 1 proton from the human body every 100,000 years. Maybe that doesn't matter to us, but on a space station those start to add up! so immortals trying to ship of Theseus their bodies and planets may fight the proton wars. Long before a sizable number of matter decays I would expect a future civilization to have already created grids of black hole farms and chucked all the rotting/useless matter in, create new planets as needed and cycle their own atoms out through cultivation breathing exercises. Or a tiered system of vaults (3km), power plants (0.1fm) and forges (0.001fm)