Which molecules cause the diffuse interstellar bands?
In plain words
Starlight crossing interstellar gas shows hundreds of absorption bands, called diffuse interstellar bands, whose source is unknown. After a century, only one molecule, the positively charged buckyball C60+, has been confirmed.
Precise statement
Of more than 500 known diffuse interstellar bands in the $4000\ \text{to}\ 13000\ \mathrm{Angstrom}$ range, only the C60+ bands (strongest near $9577\ \text{and}\ 9632\ \mathrm{Angstrom}$, weaker ones near $9365\ \text{and}\ 9428\ \mathrm{Angstrom}$) are securely assigned, by cold gas-phase laboratory spectroscopy. Identify the carriers of the remaining bands, requiring a laboratory gas-phase spectrum matching wavelengths, widths and relative strengths at interstellar temperatures.
What would settle it
Cold gas-phase laboratory spectra of candidate molecules matching the strongest unassigned bands (5780, 5797, 6284 Angstrom) in position and profile.
Status in the literature
Unverified note
Cyclic C4+ was proposed in 2025 for the 5039 Angstrom band (Krasnokutski et al., ApJ, 2025) and remains unconfirmed.