Hypermatter reactors are power generators that function by annihilating hypermatter to produce massive amounts of power.
Hypermatter is a form of matter that exists in hyperspace and is essentially tachyonic in nature. Trapped inside the realspace of the hypermatter reactors, it is annihilated as it reaches infinite speeds.
Hypermatter reactors are found in almost all manner of hyperspace-capable spacecraft, from the hyperdrive rings used by some starfighters during the Clone Wars to Imperial Star Destroyers. The Death Stars were powered by the largest hypermatter reactors ever built.