Clouds which are dusty, obscure and block background light within the galaxy, are called dark nebulae. The most famous of these known to man is the Horsehead Nebula. These dark clouds of gas and dust, which sit within the Milky Way Galaxy, only become visible when they are positioned against a background of bright stars. 370 dark nebulae (otherwise known as nebulas,) were catalogued by an American astronomer by the name of Edward Emerson Barnard, including Barnard 33 (the famous Horsehead Nebula.)
To find the Horsehead Nebula, one should look within the Orion constellation. Three stars make up Orion’s belt. Horsehead Nebula lies just beneath the leftmost belt star, Alnitak. While the Horsehead Nebula is the most famous of dark nebulae, it is not always the easiest to spot. Dark nebulae are often referred to as dark clouds and absorption nebulae which are clouds of interstellar gas and dust. These are sufficiently dense to partially or fully, obscure light from other objects and stars sitting behind them. These objects or stars, are large enough to produce a very noticeable effect.
Dark nebulae are visible in distant galaxies as well as within the Milky Way when a dark dust lane blocks or obscures part of a spiral arm. One classic example is the Black Eye Galaxy. Oftentimes, these dark areas are where dust is coalescing to form brand new stars. Dark nebulae are predominantly composed of molecular hydrogen. Small dark nebulae are sometimes called Bok Globules. These are cool clouds of dust and gas which often look very much like spherical drops of water. These dark nebulae are named after American Astronomer Bart J. Bok. These specific types of dark nebulae are thought to be the source of lower mass stars.
At one stage in time, one often wondered where all the stars went. What once was considered a hole in the sky is now known to astronomers as a dark cloud molecular cloud. This is where a large concentration of dust and molecular gas absorbs practically all visible light emitted from background stars. These dark areas which are said to be so very eerie, are deemed the most isolated and coldest areas in the entire universe. Stand outside on a dark night and follow the arc of the Milky Way, a good distance away from any city lights. You will then see ribbons and knots of darkness weaving their way through the bright star clouds.
A plethora of amateur stargazers tend to think that dark sky regions simply mark the absence of stars. Yet this is not true. Those dark areas are actually immeasurable clouds of gas and cold interstellar dust. Most of which is created from what scientists call the ‘dregs’ of deceased stars that exploded many years ago. Yet many of these dark nebulae will eventually contract, warm up and basically recycle themselves. This is done when they fall into clusters of hot new stars.When minute ice-covered dust grains scatter background starlight, dark nebulae are more noticeable. The dust grains are said to be less than 1,000th of a millimeter across and just 263 C.
Within each cloud is approximately 100 dust grains per cubic centimeter. These clouds are tens of thousands of years thick, though. Dark nebulae are tricky to find because one is basically looking for an absence of stars. Thomas Hardy called dark nebula 'deep wells for the human mind to let itself down into.'