Polycyclic Aromatic Hydrocarbons (PAH) are a class of very stable organic molecules made up of only carbon and hydrogen. These molecules are flat, with each carbon having three neighboring atoms much like graphite. At right is a space filling representation of the PAH coronene (C24 H12). The structures of a variety of representative PAHs can be seen here. These molecules are highly carcinogenic but they are also very common. They are a standard product of combustion from automobiles and airplanes and some (such as benzo[a]pyrene) are present in charcoal broiled hamburgers.
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Here is a comparison of an infrared spectrum of a distant object with the raman spectrum of auto soot. We do not claim that they have alot of cars out there but that the high temperatures present lead to the same kinds of molecules that we see in auto engines. PAHs are now commonly thought to be the carriers of the "unidentified" infrared bands (UIRs) and We now believe that PAHs or PAH cations (a PAH with an electron missing) are also the best candidates for the diffuse interstellar bands (DIBs).
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Based on essays written by the Astrochemistry Laboratory, NASA Ames (2001)