Unexplained Spectral Phenomena in the Interstellar Medium: an
  introduction
  There exists a number of astronomical spectral phenomena that have remained
unidentified after decades of extensive observations. The diffuse interstellar
bands, the 220 nm feature, unidentified infrared emission bands, extended red
emissions, and 21 and 30 $\mu$m emission features are seen in a wide variety of
astrophysical environments. The strengths of these features suggest that they
originate from chemical compounds made of common elements, possibly organic in
nature. The quest to understand how such organic materials are synthesized and
distributed across the Galaxy represents a major challenge to our understanding
of the chemical content of the Universe.
