How are dark nebulae, such as the Coalsack Nebula, typically rendered visible?

As silhouettes against a brighter background

Dark nebulae are fundamentally different from emission or reflection nebulae because they do not emit or reflect sufficient light to be seen directly; they are composed of thick clouds of dust obscuring what lies behind them. Their visibility, therefore, depends entirely on the presence of a brighter background. Whether observing the Coalsack Nebula or other dust lanes, the object is seen only as a void or a silhouette where the opaque dust blocks the light originating from more distant stars or brighter nebulae situated behind the obscuring cloud. If one were floating in deep space far from bright sources, the contrast against the black background would be absolute, making the shape of the obscuring dust cloud starkly apparent.