Neutronium, or neutron-degenerate matter, is a form of matter created by pressures sufficient to collapse an object's very atoms. It is used as fuel for annie-plants, making it a very important if not particularly expensive substance. It has a density on the order of billions of tons per cubic centimeter, and thus even in vast quantities it takes up very little volume, but the annie-plants necessary to contain it vary in proportion to the amount of neutronium they contain. Neutronium storage requires constant application of gravy fields, since it isn't stable under anything like normal pressure, and re-expands violently if containment is breached.
While vital to civilization, neutronium is quite inexpensive, since anything can be compressed into neutronium with the application of enough gravy. This means that neutronium producers make excellent disposals for hazmats and garbage. Compression into neutronium is also a standard technique for extremely large warships, such as battleplates, that wish to completely annihilate a smaller foe.