matter-antimatter is one of the largest releases of energy known. If we could harness that, it would be one hell of a power source, but we can only create very minute amounts of antimatter which is probably a good thing.
I like the concept of the ZPM in Atlantis. Also I remember a TNG episode where they said the Romulans use a quantum singularity for their power source and that's alot of power. But as far as interstellar space travel, like you said, Orion is the best we have, ion propulsion is much more efficient and faster than conventional rocketry but as for possible energies out there, like with Trek, warping space would be the best, most efficient way of crossing space, but the power it would take to warp space is enormous, they use matter-antimatter in Trek, it's probably the best bet if enough could be created to use as fuel. I can't think of anything we could do feasibly or even theoretically that would be more powerful, but I'm not a scientist. A few of the people on this forum are students in science, they can probably add more.
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