Neutronium is a material that exists primarily in the cores of neutron stars. It is a lattice of neutrons (one of the three building blocks of matter at the atomic/nuclear level) that is generated when the tremendous pressure generated by a supernova (the explosion of a large star) acts on its core, causing all its electrons and protons to merge and form neutrons (this is the same as the process of beta decay, that generates beta radiation, only in reverse). These neutrons are very densely packed, and bind with each other using subatomic particles called gluons. This makes it the hardest possible substance in the universe. The spins of these neutrons add up to generate extremely powerful magnetic fields, which have been detected by astronomers. The high density of neutronium also generates a strong gravitational field, sometimes so strong that it may cause the neutronium core to collapse in on itself, forming a gravitational singularity (colloquially known as a black hole).
Because neutronium is impervious to virtually all types of weapons fire, it is utilized by some species to construct practically indestructible structures. Some examples include:
- The doomsday machine had a hull composed of pure neutronium. (TOS: "The Doomsday Machine")
- The Dyson sphere discovered in 2369 had a shell made from carbon-neutronium. (TNG: "Relics")
- The central structure of the Iconian ziggurat on Vandros IV, in which was located a functioning gateway, was constructed of neutronium (DS9: "To the Death")
- The main door to Dominion Headquarters on Cardassia Prime was made of neutronium. ("DS9: "What You Leave Behind")
- The hull of the Think Tank vessel from the Delta Quadrant was composed of a neutronium-based alloy. (VOY: "Think Tank")