Neutronium is an element found only in Exo-Space, a dimension outside of reality. It was theorized by Blue Marvel, the discoverer of the Exo-Space, that Neutronium was formed as a byproduct of creation itself.
Blue Marvel tried to harvest it for years due to the Neutronium's potential to solve hundreds of global problems, but its atomic structure refused to cohere in normal space, until after the next death and rebirth of the Multiverse. This event affected the structure of Neutronium, changing its isometric form from seven to eight-atom groups, an "Isometry-8" form (Iso-8 for short), making it more stable, powerful, and perfectly comfortable in real space.
Neutronium is scarce in regular universes, but it's abundant in one specific reality known as "Battlerealm," a broken portion of time-space which once was the reality of Doctor Doom's Battleworld. Two Elders of the Universe, the Collector and his brother the Grandmaster, pitted themselves against each other for the possession of Battlerealm's Iso-8, and an object known as the ISO-Sphere, in a competition known as the Contest of Champions, in which they summoned people from across the Multiverse to Battlerealm to fight in the name of each of the brothers against each other.
Blue Marvel described the crystals of Neutronium as frozen spacetime. Their atoms fall naturally into structures of eight. When these used to be seven-atom groups, the isometric lattices tore themselves apart at the quantum level. The change of structure of the Neutronium into Iso-8 and Blue Marvel's theory that the Neutronium formed as a byproduct of creation itself presented the possibility that there had been in total seven instances of "multiversal renewal," and that the Neutronium's structure changed each time the universe was in a new iteration.
It has been stated that Neutronium is an incredibly powerful substance. The true extent of its capabilities is unknown, but it has shown to have different uses, like empowerment, healing, or stasis of individuals. It can even be used to bring people back from the dead.