UU-Matter is created by the Mass Fabricator by consuming EU. It is capable of making many items within the game.
The UU-Matter crafting recipes have deemed "secret" by the Industrial Craft 2 Development Team. They are disabled by default, but can be re-enabled via the toggling of a config option. The Official Industrial Craft Wiki has removed all links and user comments pertaining to showing these recipes.
All known UU-Matter recipes are documented in this article.