Warning: This is more or less the Applied Phlebotinum of Galaxy Rise. Read at your own peril.Just about every reasonable sized device (like a hoverbike and above) used by the U.E.F. utilizes what's commonly referred to as a "Pearl Reactor".
This reactor works around one item - the "Pearl". Also referred to as the core. The pearl is a solid sphere that varies greatly in size. A large capital ship may have a
giant core, but something like a car may have a tiny one. The Pearl is mined from various planets deep down, and works kind of similar to hydrocarbons. The pressure exerted during formation of the planet with just the right combination of materials will deposit an extreme amount of energy into a sphere, which is mined out and used.
Basically, the Pearl is a battery with an
extreme charge. U.E.F. reactors have the core placed in the middle of a spherical chamber, which is usually much bigger than the core. The Pearl sits right in the middle, suspended in mid-air by the reactor. The reactor carefully stimulates the Pearl with inputs of energy, and the core responds with a much greater amount of energy. Pearls last for a virtually infinite amount of time.
However, if the Pearl receives too much energy, it'll essentially start a chain reaction, releasing all the stored energy at once in a large explosion. The required input energy for this to happen is quite large, though. Therefore it must be overexerted over a variable period of time. Commonly referred to as an overload/meltdown when happening in U.E.F. reactors.
The pearl has two other disadvantages. It requires a relatively large amount of space, meaning it can really only be used for structures and vehicles, but not anything else. The second is that Pearls are relatively rare. While they do have a virtually infinite amount of charge, they are slowly lost forever in wrecks, explosions, combat, etc.
Basically:
A kinda-rare giant pearl-like sphere floats in mid-air inside a U.E.F. reactor and acts as a super-dense, slightly unstable, battery.