Æther

Æther, a.k.a. ether is the element proposed by Dmitri Mendeleev, the creator of the Periodic Table, to be the "element" that composes empty space. Sometimes called Element X, it was believed to flow through all materials regardless of the materials and was thought to be inert. Much like Neutronium, it has an atomic number of 0, although it is thought not to be made of entirely neutrons, but to be infinitely small and undetectable. Due to it being inert and Helium's supposed ability to flow through some liquids, it would be placed over Helium on the Periodic Table in a new Period 0. It was first publicly proposed by Dmitri Mendeleev in 1904.