Dark matter

Present
First speculated in the 20th century, Dark Matter was a very mysterious subject. The name refers to the only certain property of dark matter: we can't see it. It's existance is only speculated as a result of inconsistencies in our mathematical equations to describe the universe. Some say it doesn't exist at all, others say there is far more dark matter than "regular" matter.

In Futurama
By the year year 3000, Dark Matter has become widely used as a fuel source, particurlarly for space craft. Beyond this, little has been said on the subject, and we have only a few observerions. For one, it can be compressed into a hard, solid substance with an incredibly high density and weight - a ball of it no more than one inch in radius can weigh many thousands of pounds.

Dark Matter Engines
Spacecraft fueled by dark matter are fairly common in the future, as proven by the availablilty of dark matter fuel at orbital filling stations. The Planet Express Ship's dark matter engines seem to burn the dark matter in a large furnace, though exactly how this energy is translated into propulsion is understood only by the Professor and his clone, Cubert Farnsworth.

Trivia

 * Conventional methods of Dark Matter mining have not been revealed, but the Planet Express crew found an new source of dark matter - Nibbler. Nibblonians excrete pellets of pure dark matter!
 * One of the few places where Dark Matter has been known to be mined was the planet Vergon 6, now destroyed. It was completely filled with the heavy substance and imploded due to the lack of support for the surface after it was all mined out.
 * Interestingly, Hermes has stated that Nibbler produces so much fuel that the company has to build a new wing to store it. Perhaps this is one of the ways Planet Express funds itself - by selling starship fuel.

Quotes

 * Leela: Dear Captain's Diary: I may not have found love on this mission but I did find a cute little companion who excretes starship fuel. And that's just as good.