The Dark Matter are a race of dark, satanic creatures that are completely incapable of feeling the emotion we call Happiness, who used to keep to themselves as they fed on cosmic material in the empty void in which they reside. Unfortunately, pop singers from Pop Star who thought they could be cool disturbed these creatures by bombarding them with annoying garbage that was supposed to be "Pop Music." Eventually the Dark Matter's king, Zero, went insane and decided to take over the universe, starting with Pop Star, thus leading to the creation of a video game franchise known as Kirby's Dreamland, as well as the beginning of a never-ending political revolt against King Dedede, who was just trying to help his poor people. Most Dark Matter have round bodies with no noticeable limbs, and have either one or three eyes (except Miracle Matter, who has 24).
Dark Matterling: A small blob of Dark Matter that looks like a purple bubble with one eye. These are often the cannon fodders who keep swarming in Pop Star and possessing people just for the heck of it. There is also a more powerful variant that has smaller orange bubbles forming a ring around it.
Mariel: A Dark Matter creature that seems to resemble a 4-legged spider-like creature. These creatures disguise themselves as black balls until an unsuspecting tourist comes close enough, then they come out and kill their victim(s). But they can't jump, fly, or climb walls, so they have no match against any potential opponent who can do any of those things. If Kirby swallows a Mariel, it will deal damage.
Nidoo: Basically a large heap of liquid Dark Matter who, similar to Mariels, waits for a fool to get real close, then it reveals itself and tries to jump on them. They disguise themselves as doors.
N-Z: Even though thes dudes have spikes all over their bodies and can totally gore a victim to death, they never bother to be aggressive in any way. All they do is walk around and hope to bump into someone. These guys are just as easy to beat as one of those Waddle Dee things.
Gooey: A friendly blob that helped out Kirby in Kirby's Dreamland 3. See, not all Dark Matter are bad! Gooey has similar attacks to Kirby.
Zero: The king of the Dark Matter, a gigantic white ball of negative energy, and the one behind all of the Dark Matter invasions on Pop Star. He died in Kirby's Dreamland 3 when Kirby shot his eye out by bombarding him with the emotion we call love. That wasn't the last of Zero, though...
Zero Squared (or Zero-Two or 02): Zero came back from the dead sometime between KD3 and Kirby 64 and became 02. Hell bent on revenge, 02 decided to think bigger and take over the whole galaxy in which Pop Star was located, waiting for Kirby to take the bait and find 02 so they could settle their rivalry (prefferably ending with 02 consuming Kirby's soul). 02 is much more powerful than his living counterpart and not only has extra features such as wings and having three eyes instead of one, but he's also immune to the joyous feelings that had killed him previously. But Kirby managed to kill him again in Kirby 64 by randomly shooting various precious minerals (diamonds, emeralds, saphire, etc.) at his three eyes until 02 flipped over revealing a band-aid on his head (or was it his buttocks?) which Kirby shot until the bandaged wound was opened, and it got infected and 02 died.
Dark Matter Swordsman: He was sent by Zero in Kirby's Dreamland 2 to possess King Dedede and some other important people who were leaders of Pop Star, in order to weaken Pop Star's defenses and prepare it for invasion. When Kirby confronted this psycopath, the Swordsman challenged Kirby to a duel as the Swordsman had almost legendary swordsmanship skills. Kirby, however, inhaled the Swordsman and copied his abilities after spitting him out, and Kirby's superior awesomeness allowed him to win the battle. leading to Dark Matter Swordsman's death. A clone of him later appeared in Kirby Planet Robobot where he portrays a terrifingly beutiful 'stomach eye'. UNLEASH THE MADDNESS!!
Miracle Matter: Miracle Matter was essentially an elite in the Dark Matter army who looks like a 50-sided die (except he really only has 24 sides, one for each eye). He was sent as a last resort in Kirby 64 to stop Kirby from reaching 02 and to trick Kirby into thinking that Miracle Matter was the final boss. And just to make Kirby cry, Miracle Matter decided to become one of those super-annoying Kirby bosses that mimic Kirby's Copy Abilities, but then Miracle Matter had to take it a step further and abuse those Copy Abilities by being immune to any Copy Abilities from Kirby except for the one he is currently using. Of course, Kirby's sheer awesomeness allowed him to win again, killing Miracle Matter by defeating all of his alternate forms, which somehow caused him to go blind in all 24 of his eyes. Miracle Matter then died of embarrassment.
Dark Mind: A Dark Matter in the Mirror World who had conquered all of Mirror World and decided to try to take over YouTube Poop World sometime just before the events of Kirby & the Amazing Mirror. Meta Knight (yes, the Hyrulian assassin who tried to kill Kirby and Dedede on several occasions) foolishly attempted to fight Dark Mind himself, only to be beaten by a darker mirror form of himself. So, as usual, Kirby had to go in and save the universe by fighting Dark Mind. At some point, Dark Mind turned into a flaming eyeball of power or whatever, but Kirby still proved to be awesome enough to beat him in both forms.
Dark Nebula: A Dark Matter that was sealed in a chest inside Dedede's castle due to its evil power sometime prior to Kirby: Squeak Squad. But then a group of thieves called the Squeak Squad stole said chest. And when their leader, Deroach, opened up the chest—you guessed it—he got possessed and Kirby had to fight him. After beating Dark Deroach, Dark Nebula was expelled and revealed its true form and they fought. Then as Kirby was about to deliver the final blow, he noticed that Dark Nebula (whose body appeared to be a black star—boring, right?) looked a lot like Patrick Star. Kirby asked if they were related in any way (which they really weren't) and Dark Nebula died of embarrassment (just like a genie Miracle Matter!), disintegrating into a pile of dust.
Magolor: Magolor was originally an innocent wizard from a planet called Halcandra who's planet was in danger with extreme ... events... and many had to leave, although he was just 22 years old and still a senior in college. He just happened to find the Master Crown whilst exploring Pupupu-Land and stopping by at a city 200 miles away from Pupupu Village (called CappyTown in the shitty 4Kids! dub) and getting a hangover at one of its most famous bars.
Unfortunately for him, the Master Crown was actually cursed by Dark Matter, and it possessed Magolor once he put it on in Kirby's Return to Dreamland. The possessed Magolor deformed and turned into a three-eyed Dark Matter monster with enormous reality-warping power. It's a wonder how Kirby even managed to beat this guy. Thankfully, just before Magolor was about to die, the Master Crown spontaneously combusted, allowing Magolor to go back to normal, making him the first Dark Matter to have not been a Dark Matter originally, and to not have died in a fight with Kirby.
Dark Mandy: She is the most powerful female in the youtube poop universe, and the clone of Mandy, nuff said.
Shadow Squid: A male Inkling that can only use black ink. He also has a sister named Dark Squid which, like Shadow Squid, she can also use black ink.