Final answer:
Prominent dark objects observed in silhouette against a lighter background are called silhouettes; they are formed when light is directed from behind the object, creating a dark shape against the background.
Step-by-step explanation:
Prominent dark or nearly dark colored objects observed in silhouette against a light-colored background are called silhouettes. To create a silhouette, the direction of light needs to be coming from behind the subject, which is referred to as backlighting or being backlit. When this occurs, the subject blocks the light and appears as a dark shape against the brighter background. Silhouettes can be a powerful visual element in photography, as they simplify the subject to its most basic form and can convey drama and emotion. Additionally, in astronomical contexts, silhouettes can reveal phenomena such as dark nebulas, which obstruct the light from stars and are visible against the Milky Way, as in the case of Barnard 68, providing insight into regions of space that are rich in dust and gas.