One should view antimatter propulsion or energy as the ultimate battery. Why? Because antimatter needs to be manufactured. There's no natural source to mine. So

One should view antimatter propulsion or energy as the ultimate battery. Why? Because antimatter needs to be manufactured. There's no natural source to mine. So whatever energy you get out, you had to spend at least that (because of thermodynamics, it's really more) to make the antimatter in the first place. So where do you get that energy in the first place? Everything leads back to solar power. In this case, since we're talking about far-future tech, we return to what I consider the most likely path for humanity: the Dyson Swarm. This is the sort of thing you can do with a truly mind-bogglingly large energy budget. And this matters because the energy budget, regardless of the energy source, for interstellar travel, is so ridiculously large.