Barnard's initial catalog of dark nebulae, first published in 1919, described 182 entries and was superceded by a 1927 follow-up publication which was expanded to include 349 entries.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: Dark nebula B133 is one of various notable such nebulae in Aquila. It lies in the southern portion of the constellation and near the border with Scutum. Other Barnard dark nebulae within the immediate vicinity include B127, B129, B130 and B134 as well as the bright carbon star V Aql. This nebula is elongated and generally runs northeast to southwest measuring approximately 10' by 3' and with the southern half being much darker and denser than the northern half. As noted by Barnard, this nebula has a general cometary form and convex west side.