Neutronium

Neutronium is a rare dense material, found naturally in the cores of neutron stars. It is impossible to scan inside neutronium. Despite many readings and theories, Starfleet has never able to produce neutronium.

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. 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.

Fun Facts:

• The Borg have never assimilated the knowledge to collect or create neutronium.

• In 2369, the USS Enterprise, NCC-1701-D encountered the only known Dyson sphere. It was composed of a carbon-neutronium alloy and was impervious to phaser fire.

• The Iconians had mastered the use of neutronium, and used it on a massive structure that contained 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.

• The Dominion had begun using neutronium by 2375, and used it to protect the entranceways on Dominion Headquarters from weapons fire.

• In 2371, the Voyager encountered an artificial asteroid composed of a neutronium alloy, created by the Vidiian Sodality.