UU-Matter is an end-game Industrial Craft 2 item that is created in a Mass Fabricator. The more power provided to the Mass Fabricator, the quicker it will produce UU-Matter. Inserting Scrap in the bottom slot of the Mass Fabricator will speed up the process of creating UU-Matter by up to 6 times, the duration of which is shown in the GUI. It takes 1,000,000 EU to generate one UU-Matter, or 166,000 with Scrap.
Many items can be crafted with UU-Matter. A special item, Iridium Ore, can be only obtained by crafting it from UU-Matter. Sometimes, however, it's more efficient to craft one thing and then convert it to another rather than crafting the desired item directly. For example, snow is cheaper to craft through blocks than snowballs. The blocks can then be converted to snowballs, saving UU-Matter. With the use of a Minium Stone, it's cheaper to make Gold Ore and turn it into Diamonds than if one created Diamonds out of UU-Matter directly.
Another use for UU-Matter is in the crafting of water and lava source blocks. Even if placed, the source blocks will not flow like normal water or lava. These blocks can be placed in the Nether, unlike the normal buckets of water or lava. Source blocks will begin to flow if a block is updated nearby, so placing anything around them is ill-advised.