Futurama Wiki says:(Dark matter) is the densest, and therefore heaviest, substance in the universe, with each ball that Nibbler produces weighing as much as 1000 suns.
Professor Farnsworth says dark matter is so dense that
"each pound of it weighs over ten thousand pounds."
Nibbler himself admits
"We live long and are celebrated poopers"
He has an enormous appetite. How much food does Nibbler require in order to produce one of his dense 1 pound=10,000 pounds power packed pellets of dark matter?
From the Direct-to-DVD film Bender's Game, the Nibblonians were in cages being forced to defecate; and were fed 4 chickens in between bowel evacuations. There was also prune juice on the wall.
Seeing as how it was Mom that was caging them and harvesting their dark matter, it is likely that this is all they needed in order to produce one dark matter "pellet". (Mom is notoriously ruthless and stringent.)
In the episode when Nibbler first meets the gang, he eats all of the animals that were collected by the Planet Express crew:
Hermaphlamingos
x2 and after all of that he only produced one ball of dark matter. (Source for list of animals)
