Red matter is an artificial material created by the Vulcan Science Academy in the 2380's. Manufacturing red matter requires an extremely rare mineral called "decalithium".
When activated, red matter creates a black hole (or something vaguely similar to one), which then consumes any nearby matter to increase its mass. A red matter singularity can destroy a planet in a few minutes and can even consume material from an exploding star. A tiny volume of red matter is sufficient to start the process, and larger amounts do not appear to increase its potency. For unknown reasons, the black holes created by red matter also function as wormholes through which a ship can travel through time and space.