UU Matter - TinyModularThings/IC2Classic GitHub Wiki
Universal Usable Matter (or UU-Matter) is an exotic type of matter that can be produced in a Mass Fabricator.
Little is known about its use, but there are multiple reports, that one can use it for crafting to create all kinds of items and blocks, some even claim to be able to create diamonds using it. Using only UU-Matter inside a normal crafting table, creating various patterns in the three by three crafting window is able to create many different raw materials, like stone, mob drops, lava etc. The recipes are hidden in JEI, so you need to experiment a bit.
Be aware, UU-Matter is the only way to get Iridium Ore, which is a crafting component for the most advanced items and blocks.
The hiding of UU-Matter recipes can be disabled in the Config using "EnableSecretRecipeHiding=false" but this isn't recommended.