^Except most of the references to neutronium, especially its use in DS9, make no sense. Neutronium is the stuff neutron stars are (partly) made of. It's so dense that a spoonful would have the mass of a mountain. Its own gravity would pull it into a sphere, so it couldn't have the shape of the Doomsday Machine. And if you made a door out of the stuff, like the door to the Dominion stronghold in "What You Leave Behind," it would instantly plummet through the planet's crust and sink to the center. That is, if such a small amount of neutronium could stably survive at all, which it couldn't, because its own internal degeneracy pressure would cause it to explode back into normal matter.
Which is why, in The Buried Age
, I explained the references to "neutronium" as actually being "hyponeutronium" (a term I cribbed from Diane Duane), a nickname for an alloy of dense, stable transuranic elements with nuclei made mostly of neutrons.