Barnard's initial catalog of dark nebulae, first published in 1919, described 182 entries and was superceded by a 1927 follow-up publication which was expanded to include 349 entries. These "dark nebulae" are believed to be the result of molecular clouds of dust and gas which are present in our line of sight and which absorb the starlight originating behind them, thus providing the earth-based observer working in the visible spectrum the illusion of a black "void" (these dark nebulae are NOT associated with dark matter!). Some of the best-known examples of these "absorption nebulae" include B33 (Horsehead Nebula), B68 (Molecular Cloud 68), B72 ("S" or Snake Nebula) and B142-143 ("E" Nebula).

Note: The dark nebula B143 is located within three degrees northwest of Altair (α-Aql, mag 0.93) and is accompanied by a large number of other Barnard dark nebulae in the immediate vicinity including B142, B340, B334, B336 and B337 all within a two-degree circumference of B143. As noted by Barnard, this particular nebula has the general shape of a square approximately 0.5 degrees wide and with the west side missing. Along with B142 which lies immediately to the south, the combination of B142 and B143 are commonly referred to as the "E" nebula owing to the fact widefield views reveal a combined formation which is similar to the letter "E".