Dark Matter Thieves

The Dark Matter Thieves (暗黒盗賊団ダークマター, Ankokutōzokudan Dākumatā) are a group of alien invaders led by the powerful Boros that attacked various planets to find valid opponents.

Headquarters
The Dark Matter Thieves headquarters is a giant spaceship, so big that it can obscure even A-City. It's heavily armed with cannons able to shoot enormous destructive bullets and is able to fly thanks to a Power Core, the destruction of which makes the spaceship fall almost immediately.

Alien Conquerors Arc
The Dark Matter Thieves arrived on Earth and entirely destroyed A-City, with the exception of the Hero Association headquarters. Melzalgald was the first to leave the ship and kills the Sky King and his henchmen. Melzalgald then tries to kill A-Class hero Iaian, but failed and the two began to fight. At the headquarters, Saitama managed to jump from the rooftop and is shot by colossal-sized bullets, but was able to dodge and repels back. Iaian is still fighting Melzalgald and while dodging an attack, his left arm is torn completely. Melzalgald then begins splitting itself into five different forms. Feeling bored of his situation, the monster is suddenly cut to pieces by Atomic Samurai.

Atomic Samurai is then joined by Metal Bat, Bang and Puri-Puri-Prisoner to fight Melzalgald. While Melzalgald was fighting, he decided to contact the ship to prepare cannons. He creates a small flying head to contact the ship, but the head was struck by Metal Bat. He then received a distress call from Geryuganshoop, who tells him an intruder got on the ship and that Groribas had been defeated.

Trivia

 * According to Genos' notes, the size of the Dark Matter Thieves' ship is 9,230m x 15,024m.