It was explained in the episode “Love’s Labours Lost in Space” that Nibbler lived for many years on a distant planet called Vergon VI where it consumed all of the world’s fauna (Nibbler is a notorious voracious eater, capable of swallowing giant creatures whole). Likewise, Nibbler also pooped out so much dark matter that he essentially filled the entire interior of the planet. This would set the precedent for “Bender’s Game,” which saw the villainous Mom (Tress MacNeille) kidnapping Niblonians and force-feeding them chickens to get them to defecate more. Dark matter fuel is, naturally, quite valuable.
Mom’s poop farming factory was, in “Bender’s Game,” located on Earth. This location was handy for story streamlining, as Horstead wanted it to be located on the Nibblonian homeworld. Cohen recalled the change, saying:
“The original form of this story was quite different than it ended up being. In the first draft as written by Eric Horstead. […] A lot of the action was going to take place on Nibbler’s home planet, which makes sense because that’s where the Nibblonians live … So obviously the center of Nibbler’s planet was gonna be all dark matter and Mom’s dark matter mine was going to be on that planet and they were gonna investigate that mine, but it just got too complicated and we said, ‘Let’s keep it on Earth,’ and it saved about half an hour of explanation.”
In a movie that exists merely to transpose the “Futurama” characters into a comedic fantasy world, all the extra investigation would have merely delayed things. MacNeille joked that they couldn’t get the drilling rights on the Nibblonian planet.
Incidentally, “Bender’s Game” is a savvier and funnier spoof of fantasy tropes than Groening’s story-heavy series “Disenchantment.” Although fans would clearly welcome a crossover.