The Galactic Center was a supermassive black hole which was in the exact center of the galaxy. It was first discovered in the Sith Wars by a Sith Armada, which was quickly sucked in and was unable to warn anyone of the black hole. Later on in the Galactic Civil War, the black hole was found by some Rebel scouting ships, but they were also unable to report the hazard. Later on, the Yuuzhan Vong and several Jedi ships engaged each other in battle, and the Jedi fled to the Center. There, the Jedi were able to report it to the nearby Imperial Fleet which was in an alliance with the New Republic, but the Yuuzhan Vong weren't warned and were sucked in. After that, the Imperials kept the hazard a secret.