Star Wars - The Old Republic
Neuranium was a metallic substance used to shield from radiation, and one of the heaviest and densest metals in the galaxy. A millimeter of it could stop most scanners, and it could even temporarily block lightsaber energy. However in order to cover a space as large as a cargo hold, it would be prohibitively heavy and thus inefficient to use. Neuranium was so dense that some gravity-sensitive species noted its warping of the space-time fabric of gravity itself that a large piece created.