Neutronium

Neutronium is a rare dense alloy, found naturally in the cores of neutron stars. It was impossible to scan inside neutronium. (TNG: "Evolution"; TOS: "The Doomsday Machine") Despite many readings and theories, Starfleet was never able to produce neutronium. (VOY: "Think Tank")

• The Borg never assimilated the knowledge to collect or create neutronium. (VOY: "Think Tank")

• In 2267, the sensors of the USS Enterprise recorded a pure neutronium hull on the extra-galactic doomsday machine prior to its destruction. (TOS: "The Doomsday Machine")

• In 2366, Doctor Paul Stubbs studied the cyclical decay of trace amounts of neutronium in the Kavis Alpha sector through the use of the probe "The Egg." (TNG: "Evolution")

• In 2369, the USS Enterprise-D encountered the only known Dyson Sphere. It was composed of a carbon-neutronium alloy and was impervious to phaser fire. (TNG: "Relics")

• The Iconians had mastered the use of neutronium, and used it on a ziggurat containing a gateway on the planet Vandros IV. This was discovered by the Dominion in 2372, and was destroyed by a joint Starfleet-Jem'Hadar team before a group of rogue Jem'Hadar could use the gateway. (DS9: "To the Death")

• It is unknown if any neutronium was present on Iconia as seen in TNG: "Contagion".

• The Dominion had begun using neutronium by 2375, and used it to protect the entranceways on Dominion Headquarters from weapons fire. (DS9: "What You Leave Behind")

It is assumed, though unclear, that neutronium is a harder substance than rodinium since neutronium seems to be impervious to weapons fire while rodinium can be destroyed by Romulan plasma torpedoes. However, according to Spock, rodinium was the hardest substance known to Federation science as of 2266. (TOS: "Balance of Terror") Neutronium was mentioned in 2267 during the confrontation with the Doomsday Machine. So, it is possible that neutronium was first discovered by the Federation sometime between those two incidents.

Neutronium in the Delta Quadrant

• Separated from the Alpha Quadrant in 2371, the USS Voyager encountered neutronium several times while out-of-contact with the Federation.

• In 2371, the Voyager encountered an artificial asteroid composed of a neutronium alloy, created by the Vidiian Sodality. (VOY: "Phage")

• According to an Alpha Hirogen hunter, at least one collapsed star in the Delta Quadrant had a neutronium mantle. The hunter claimed to the crew of Voyager that he tracked a silicon-based lifeform through this mantle. (VOY: "Prey")

• In 2375, Voyager encounter the Think Tank, who traveled aboard a vessel composed of a neutronium alloy. This impressed the crew of Voyager, who had rarely encountered it before. (VOY: "Think Tank")

It is possible that the Think Tank retrieved knowledge of neutronium as payment for curing the Vidiian phage.