Red matter is a highly unstable substance that can form an artificial black hole, used by the Vulcan Ambassador Spock in a failed attempt to stop the Romulan supernova.
In the 24th century, a supernova threatened to destroy Romulus. Spock prepared to ignite red matter inside the exploding star so that a black hole would consume it, but he arrived too late to save the Romulan homeworld. The singularity drew both his ship and the Romulan mining vessel Narada into the past.
The Narada's commander, Nero, blamed Spock and the Federation for the loss of his world. In the altered timeline he used Spock's red matter to destroy Vulcan, before the Narada was itself consumed when the remaining red matter was ignited.