Here's something more interesting. A matter-antimatter reactor converts 100% of mass to energy so it's a hundred times more efficient than fusion. In modern times antimatter has been produced at quantum levels in large accelerators such as the Hadrian collider. So it does in fact exist and can be produced.

However a matter-antimatter reactor has some serious technical problems. For one it's currently impossible to create antimatter in any practical quantity. Second if antimatter comes in contact with matter, instant boom. Like a sugar cube size of the stuff could level a large city. So containment would be an insurmountable problem.

The interesting part is when you see an antimatter reactor in shows like Star Trek, it's based on real science. Interestingly in 1968 when they wrote the original Star Trek, nobody knew antimatter was a physically real thing. That's a case of sci-fi predicting science.