Power Core

The Power Core (動力球, Dōryoku-kyū) was the power source of the Dark Matter Thieves' mother ship.

Alien Conquerors Arc
The power core keeps the giant spaceship hovering above A-City. After Saitama defeats Boros, the power core shatters, causing the ship to crash.

Abilities
As the power core was the source of power for the giant spaceship, it can be assumed that it could supply immense amounts of energy.

According to Boros, the mother ship will not crash down as long as the Power Core stays intact, regardless of how much damage the ship takes. This suggests that the source of the ship's levitational ability and all of its components has its roots set in the Power Core.