Dark nebulae are large areas of gas and dust that are not illuminated by neighboring stars. They can only be identified because they block the light of more distant stars. Therefore, the densest, or most extended nebulae, show up as „black holes“ in the sky – but they have nothing to do with real black holes. These are extinguished and collapsed stars that cannot be directly observed. Dark nebulae can be found only in the region of the Milky Way. If you look out of the plane of the Milky Way, there are not enough stars to be obscured – and if they did, there wouldn‘t be enough contrast with the few adjacent non-hidden stars. The dust itself remains invisible, you can only see the silhouette against the background stars.
The famous Horsehead Nebula in Orion is one of the most well-known dark nebulae, but remains reserved to telescopes and photographs. The Pipe Nebula in the constellation Ophiuchus can be seen in binoculars, and from the southern hemisphere you can also see the Coal Sack in the constellation of the Southern Cross. Even the „Great Rift“ of the Milky Way (the „Northern Coal Sack“) between Deneb (α Cygni) and α Centauri is caused by dense, dark clouds that impede the view of more distant stars.