Why do dark nebulae appear as silhouettes against stellar backgrounds?

They are so dense with dust and gas that they effectively block light from stars located behind them.

Dark clouds, also technically referred to as dark nebulae or absorption clouds, achieve their visual effect not through emitting or reflecting light outward, but through absorption and extinction. They possess a sufficiently high concentration of dust and gas molecules that they act as opaque screens positioned between the observer and the luminous sources lying beyond them, such as distant stars or brighter emission nebulae. This density causes the light originating from the background to be completely obscured, resulting in the perception of a dark patch or silhouette superimposed on the bright stellar backdrop of the galaxy.