Void / Vacuum
Gases & OtherVacuum or void region: no material, and particles stream through without interacting.
Where these numbers come from
Not a material: void regions carry no composition.
No material card
MCNP writes material 0 with no density on the cell card. Serpent uses the keyword `void`, OpenMC leaves `Cell.fill` as `None`, and SCONE uses the reserved material name `void`. None of them wants a material card, which is why this entry has no composition.