Posonium is a material found in some -verses, sometimes artifically, that occupies all of the energy states of its component fermions, simultaneously. This makes it infinitely energetic, which limits its general usefulness; many universes can not support its existence, with either the universe or the posonium collapsing immediately upon introduction. Even universes that support it often have it diffuse and spread out immediately, filling the universe with an infinitely dense posonium fluid as the laws of physics attempt to lower the energy level to no avail.
Another consequence of this is that posonium, in a stable state, is completely impenetrable to fermionic matter; the Pauli exclusion principle does not allow this since all possible energy levels are filled.