Unobtainium: Of many kinds. Sophisticated materials science is one of the major areas of advancement in this particular universe. Of particular note: deuterium slush, metastable metallic hydrogen, helium-3 and antimatter (more specifically, antideuterium slush) for power, room-temperature superconductors, sapphiroids (the trade name for the high-grade kind is Adamant™ – not adamantium, because it’s not an element; after all, transparent aluminum has been used, even if accurate), carbon nanotubes, highly refractory cerametals and metallic glasses, muon metals, strangelets, raw tangle – oh, and fun nonbaryonic things like exotic matter (you make stargate frames out of it), gluonic string (held together by the strong force, thus with the best tensile strength available), and so on and so forth. Less elementally, various nanofluids with fascinatingly exotic behavior, nanotech composites, and smart and biomimetic materials (living metal, nanowell-bearing programmable matter, etc.), computronium (okay, that’s not an element either, but…), and again, and so forth.
And medically speaking, of course, immortagens.