Matter in which the electron shells are collapsed to form and extremely dense, very strong material. Neutronium must be kept in an intense gravitational field to prevent the electron shells from expanding explosively. The shell of the Dyson Sphere is constructed of carbon-neutronium. It is unknown how the Builders contained the neutronium, as the artificial gravitational field in the Sphere seems to be insufficient to keep the electron shells collapsed. The only other artificial neutronium discovered to date was the so-called Doomsday Machine encountered by the USS Constellation NCC-1017 in 2267.

Neutronium is a term originally used in science fiction and in popular literature to refer to an extremely dense phase of matter composed primarily of neutrons; historically, the word was coined by scientist Andreas von Antropoff in 1926 (scil., before the discovery of the neutron itself) for the conjectured 'element of atomic number zero' that he placed at the head of the periodic table. However, the meaning of the term has changed over time, and from the last half of the 20th century onward it has been used legitimately to refer to extremely dense phases of matter resembling the neutron-degenerate matter postulated to exist in the cores of neutron stars.