Dark nebula
Also called: Barnard object, absorption nebula.
A dense cloud of dust seen in silhouette against the stars or glowing gas behind it.
Dark nebulae are holes that aren't holes: thick dust blocks the light of everything behind them. The Coalsack, next to the Southern Cross, is visible to the naked eye; the Horsehead Nebula (Barnard 33) is a dark pillar against the faint glow of IC 434.
E. E. Barnard catalogued hundreds of them in the early 1900s — hence names like B 33.
At the eyepiece
The Horsehead is a famous challenge: it needs a dark sky, at least 8 inches of aperture and an H-beta nebula filter to darken the sky around IC 434.