Silent, dark, shield-shaped object.

The object was shaped what I could best describe as a shield-like shape. Rounded at the front, with a round protrusion at the rear. It was completely dark with no lights that I could see. I could see it because it blotted out the sky and stars. I would estimate the altitude at about where airliners fly. Maybe 30,000 feet. It was very large. Estimate at least 500x500 ft. It was completely silent and was flying at a high rate of speed. It was travelling almost directly from north to south and only took about 5 seconds to travel from horizon to horizon.