A series of absorption bands near 3.4 μm has been observed towards bright infrared objects which are seen through large column densities of interstellar dust.

These aliphatic hydrocarbons carry the -CH2- and -CH₃ functional groups in the abundance ratio CH₂/CH₃ ~ 2.5, and the amount of carbon tied up in this component is greater than 4% of the available cosmic carbon. The 3.4 μm band has been detected in absorption along more than two dozen lines-of-sight throughout the diffuse interstellar medium of our own Galaxy, as well as in the dusty spectra of other galaxies. Aliphatic hydrocarbons are also seen in the acid insoluble residue and organic extracts from carbonaceous meteorites, and the strong similarity between these features and those found in the diffuse interstellar dust, amid other signatures of a stellar history, suggests that interstellar organic material survives the formation of planetary systems.

Comparisons of laboratory residues to the interstellar dust features provide clues to the origin of the aliphatic component. The evidence supports an origin in the outflow of carbon stars, as well as a formation site in the diffuse ISM itself, rather than an origin within dense molecular cloud ices. Perhaps the most significant advance in the field comes from nearby ultraluminous galaxies which sometimes show greater potential for band profile studies than sources within our Galaxy. The redshift to wavelengths further removed from atmospheric interference makes ground-based observations of the entire L band of these galaxies feasible for z<0.3.