- "What you're seeing is the accretion disk, it's matter trapped inside the gravity well. You can't see the black hole itself."
- —Rodney McKay
A black hole is a concentrated singularity of gravity caused by a dead star collapsing in on itself, which absorbs everything within its reach, including light. Black holes cannot be seen, but the accretion disk, which is the flow of matter being pulled by the black hole, is visible. As one gets closer to a black hole, time appears to slow down, but since time is relative, it is only slow compared to those who are viewing it at a "normal" rate. (SG1: "Singularity")
While dialing P3W-451, the SGC was caught by a black hole consuming the dialed planet. If the connection had remained active, Earth would be swallowed by the black hole. As the black hole got closer, time slowed down, causing the SGC to lose contact with the rest of the outside world. The connection was broken when an explosive device was detonated close to the Stargate. This caused the wormhole to jump to another nearby gate. (SG1: "A Matter of Time")