What carrier produces the 2175 Angstrom extinction bump?
In plain words
Interstellar dust absorbs ultraviolet light most strongly near a wavelength of 2175 Angstrom. The bump's position barely changes from place to place while its width does, and no material reproduces both facts.
Precise statement
Identify the carrier of the 2175 Angstrom extinction feature (candidates: graphitic carbon grains, PAH mixtures, hydrogenated carbon, carbon onions) and explain why its central wavelength is constant to about 1 percent across sightlines while its width varies by about 20 percent or more.
What would settle it
Laboratory or ab initio optical spectra of a carrier population reproducing the fixed peak, variable width and observed strength per hydrogen atom.
Status in the literature
JWST detected the bump in a galaxy at $z \sim 6.7$ (Witstok et al., Nature, 2023), requiring the carrier to form within about 1 Gyr.