Photonium is a type of matter composed entirely of photons that are collectively trapped by subatomic forces but not closely enough to collapse into a kugelblitz[1]. Despite not producing any light, it can be classified as an element of light matter and can be found as large black spheres within the Lightverse.

Although near-useless as a fuel source or building material, photonium gains mass-energy on exposure to additional photons and can resemble a black hole without the gravitational effect. Coating a spacecraft or object in photonium thus renders it almost completely undetectable.