Why do dark nebulae appear as black patches against the sky?

Dark nebulae appear as black patches against the sky because they are composed of very dense, cold molecular clouds containing significant amounts of cosmic dust. This dust is highly effective at absorbing stellar radiation, which prevents the light from stars located behind these clouds from reaching the observer.12

Because these clouds are opaque to visible light, they create the appearance of irregularly shaped voids or "black patches" in the sky. While starlight cannot penetrate these dense layers, the clouds are not truly empty; they are the sites where new stars are formed through gravitational collapse. Astronomers can study the conditions within these cold, dark regions by observing infrared radiation, which is capable of penetrating the dust layers that block visible light.123