It's not rocket science...at least rockets as we know them.

So, why haven't we built a matter-antimatter reaction engine? The problem with developing antimatter propulsion is that there is a lack of antimatter existing in the universe.
For now, we will have to create our own antimatter. Luckily, there is technology available to create antimatter through the use of high-energy particle colliders, also called "atom smashers." But these high-energy particle accelerators only produce one or two picograms of antiprotons each year. A picogram is a trillionth of a gram.

It states that anti-matter propulsion is the most energy efficient propulsion. I suspect that will be true as long as the process of making the anti-matter is itself efficient enough to make it feasible.