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. Melzargard was the first to leave the ship and kills the Sky King and his henchmen. Melzargard 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 Melzargard and while dodging an attack, his left arm is torn completely. Melzargard 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 Melzargard. While Melzargard 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. Geryuganshoop orders Melzargard to return to the ship and take care of an intruder as he taken out a large sum of their equipment and workers on board. Melzargard states that he cannot at that moment and asks for support from the cannons. One of the heads attempts to fly to the ship, but Metal Bat knocks it out of the sky, smashing it apart over and over. After a few strikes, Metal Bat notices a small marble object in the head, and crushing the marble, the head begins to liquefy. On the ship Geryuganshoop contacts the cannons afterward, while also telepathically contacting Saitama, warning him not to go further into the ship. Saitama blatantly disregards the order and does the opposite, charging into Boros' throne room. Geryuganshoop attacks Saitama, hurling a rocks at him using telekinesis, but he is unaffected. Saitama retaliates by throwing a rock back at him with enough force to split his head in two.

Boros, was impressed by Saitama and explains the reason he attacked A-City was because of a prophecy that foretold he would find a worthy opponent. Annoyed, Saitama punches Boros, berating him for destroying A-City for a stupid reason. This however caused Boros' armor to shatter, unleashing his true strength and prepares to fight Saitama, going all out. On the ship one of the henchmen prepares to launch the cannons on the heroes, but the shells were stopped mid-air by Tatsumaki and sent back towards the ship causing it to explode. Melzargard grows larger and attempts to attack again, but his strikes are stopped by Puri-Puri-Prisoner allowing Bang to take out one of the two remaining heads. Melzargard retaliates by hitting Bang directly, sending him flying through the air, bouncing off of debris and into a concrete wall. Melzargard boast about his strength, but was surprised to see Bang is still alive, Atomic Samurai cuts Melzargard into many pieces, and Melzargard attempts to regenerate himself. Bang catches the marble inside Melzargard's head, destroys the marble, killing Melzargard's final head. Saitama and Boros continue fighting on the ship with Boros beginning to gain an upper hand firing a laser from his chest and then knocking Saitama down from behind. Boros seemingly believes that Saitama took damage from his attack. He boastfully explains to Saitama about his regenerative capabilities, remarking on how eventually Saitama will get tired and lose. Saitama simply tells him to shut up, asking him if Boros was done with the fight, only to have Boros get mad and unleash his Meteoric Burst on Saitama. Due to his powerful kick, Saitama was blown away towards the moon. Saitama return to Earth on to the ship from the moon and, due to powerful impact, the ships take a critical damage. Saitama tells Boros to give him all he's got. Boros accepts his offer. He released his most powerful trump card, Planet Buster Roar Cannon. With the powerful blast coming toward him, Saitama uses his Serious Series: Serious Strike. Saitama finally defeated Boros. While slowly dying, Boros told him that he cheated during the fight because he never used his full power against him. Saitama replied that he gave his best too but not half yet. Then Lord Boros died in vain because the prophecy was wrong as Saitama was too strong. Later the ship crashed down and was completely deconstructed by Metal Knight.

Trivia

 * The giant labyrinth in the Dark Matter Thieves spaceship, was credited to Kazuo Nomura, the creator of the world's most difficult maze.
 * According to Genos' notes, the size of the Dark Matter Thieves' ship is 9,230m x 15,024m.