A series of diffuse bands (of interstellar origin) were recorded on photographic plates early in the century. We now know of well over 100 such bands present in the UV, visible and near IR regions of the spectrum. Some authors suggest that 10% of cosmic carbon may be in the molecules that cause these features. Identifying the carriers of these diffuse interstellar bands (DIBs) has become the classic (astrophysical) spectroscopic problem of the 20th Century. Since their discovery in the early 1900's the DIBs have successfully challenged spectroscopists, astronomers, and physicists. During this time so many suggestions have been made that the subject fills many books. We think that the DIBs are caused by polycyclic aromatic hydrocarbons (PAHs) or most likely their cations, since PAH ions of all sizes absorb in the visible and near infrared, and such molecules are expected to be ionized by the intense UV field present in much of the interstellar medium. The table below shows the similarity between the DIBs and spectral data on PAHs.
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Based on essays written by the Astrochemistry Laboratory, NASA Ames (2001)