Dark Matter Thieves

The Dark Matter Thieves (暗黒盗賊団ダークマター, Ankokutōzokudan Dākumatā) were a group of alien invaders led by Boros.

Headquarters
The Dark Matter Thieves traveled in a giant spaceship so massive that it was able to obscure the view of A-City from above. It was heavily armed with cannons that could shoot enormous destructive shells and was powered by a Power Core, the destruction of which made the spaceship fall immediately.

Alien Conquerors Arc
The Dark Matter Thieves arrive on Earth and essentially destroy the entirety of A-City, except for the Hero Association headquarters. Melzargard is the first one to leave the ship and kills the Sky King and his henchmen. He then tries to kill the A-Class hero Iaian, but fails and the two engage in combat. At the Hero Association headquarters, Saitama jumps from the rooftop and is shot with colossal bullets, but easily dodge them. Iaian is still fighting Melzargard but is overwhelmed, and as in just a single attack, his left arm is blown off. Melzargard then begins splitting itself into five. Feeling bored of his situation, the monster is suddenly cut to pieces by Atomic Samurai. Metal Bat, Bang and Puri-Puri Prisoner join Atomic Samurai to fight Melzargard. While Melzargard was fighting, he decided to contact the ship to prepare the cannons. He sends one of his heads to contact the ship, but it's struck down 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 the intruder as he has taken out a large sum of their equipment and workers on board. Melzargard refuses, saying that he cannot at that moment as the S-Class heroes he's fighting are persistent and asks for support from the artillery. Meanwhile, after a few strikes, Metal Bat notices a small marble object in the head that he is fighting and seizes it. He questions Melzargard on what it is and after doing so, crushes the marble, making one of Melzargard's heads begin to liquefy. Back on the ship, Geryuganshoop contacts the artillery crew, 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 rocks at him using telekinesis, but Saitama is unaffected. Saitama then retaliates by throwing a rock back at him with enough force to split his head in two while telling Geryuganshoop that using psychic power to just throw pebbles around like that is a waste.

Boros, impressed by Saitama's strength, 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's armor to shatter, unleashing his true strength. He then prepares to fight Saitama. The Dark Matter Gunner fires the cannons onto the heroes and Melzargard fighting on the ground, but the shells were stopped in mid-air by Tatsumaki and sent back towards the ship. Melzargard then 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 managing to land a hit on Bang, sending him flying through the air, bouncing off of debris and into a concrete wall. Melzargard boasts about how weak the humans are, but is then shocked to see Bang still alive. While he's distracted, Atomic Samurai slashes Melzargard into many pieces, and as Melzargard attempts to regenerate himself, Bang catches the last marble inside Melzargard's head, and destroys it, killing it. Saitama and Boros continue fighting on the ship, with Boros seemingly gaining an upper hand firing a laser from his chest and then knocking Saitama down from behind. Boros boastfully explains to Saitama about his regenerative capabilities, remarking on how eventually Saitama will get tired and lose, while Boros will keep on healing himself. Saitama tells him to shut up, asking him if Boros was done talking, only to have Boros get very angry and unleash his Meteoric Burst on Saitama. With a very powerful kick, Saitama is sent hurtling to the moon. Saitama jumps back from the moon and lands back on the ship, causing heavy damage to the ship and literally tilting it to the point that it's touching the ground. Infuriated, Boros decides to go all out on Saitama but is met with a heavy punch to the gut by the hero, followed by a Consecutive Normal Punches, which smashes him into pieces, but Boros immediately regenerates. He then releases his trump card, Collapsing Star Roaring Cannon, stating that he will send the planet straight to hell, causing Saitama to use his own trump card, Serious Series: Serious Punch  which counters Boros's attack and almost kills Boros. While slowly dying, Boros states that it was a good fight. Saitama replies that it sure was, but Boros claims he's lying, as he was still holding back. Boros dies peacefully, finally achieving his dream of experiencing the thrill of a true battle.

The ship crashes down after the destruction of its core during the fight between Boros and Saitama; it is retrieved by Metal Knight and is moved elsewhere.

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.
 * Murata stated that it takes around 4 hours to draw one panel of the spaceship (round up to 6 hours when including the sketches) as a fun fact on his stream. He did not let his assistant handle this part and he kept redrawing.
 * The ship is made from a special metal alloy not found on Earth.
 * The ship is so resistant that it didn't fall even after a series of Saitama's "normal punches".
 * Saitama was the first intruder to reach Boros's throne room.