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 the powerful Boros that attacked various planets.

Headquarters
The Dark Matter Thieves travelled in a giant spaceship, so big that it can obscure even A-City. It is heavily armed with cannons than can 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 arrive on Earth and essentially destroy the entirety of A-City, with the exception of 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 is able to 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 an 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 Melzalgard on it is, leading Meal into crushing the marble, the head begins 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 he is unaffected. Saitama 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 this 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' armor to shatter, unleashing his true strength, and he then prepares to fight Saitama. The Dark Matter Gunner fires the cannons onto the heroes and Melzardard fighing 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 with the fight, 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 Saitama, followed by a consecutive normal punches, which smashes him into pieces, but he regenerates. He then releases his trump card, Collapsing Star Roaring Cannon, stating that he will send the planet straight to hell, causing Saitama uses his own special move, Serious Series: Serious Punch  which counters Boros' attack and almost kills Boros. While slowly dying, Boros states it was a good fight, Saitama replying 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 as 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.