Power Core

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

Alien Conquerors Arc
The power core kept the giant spaceship hovering above A-City. After Saitama defeated Boros, the power core shattered, causing the ship to crash down in an explosion.

Abilities
As the Power Core is the source of power for the mother ship, it can be assumed that it can 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.