Mithranium

Mithranium is an elemental metal found beyond Uranium on the periodic table.  In its purified form it appears silver/white and is somewhat malleable, it can be combined in a number of ways to make useful alloys and substances.  Mithranium does not occur naturally and can only be created by the massive energies of planet spanning Titan Colliders.  Once produced, Mithranium is extremely stable, with a half life thought to be in the hundreds of billions of years.  This means all existing Mithranium and its rocky ore Mithrite were once artificially fabricated by intelligent beings.  As hundreds of billions of years pass, the amount of Mithranium grows until it is fairly ubiquitous throughout the universe, even finding its way into most stars.  Still, it is relatively rare and coveted beyond any substance in the known universe, its alloys find their way into most major technology in the 224 MM, and it functions much as gold did in humanities past.

Mithranium Alloys

Mithrite - Rocky Silicone/Oxygen/Mithranium Ore from which pure Mithranium metal is refined.

Mithrene - Carbon/Mithranium substance which mimics the structure of Graphene with Mithranium binding the layers together.  Creates flexible but practically unbreakable substance.  Also a room temperature super conductor.  

Mithrete - A concrete like substance made from Calcium and Silicone replacing Mithranium with Aluminium to create an almost indestructible building material.