  The recent spectacular progress in the experimental and theoretical
understanding of graphene, the basic constituent of graphite, is applied here
to compute, from first principles, the UV extinction of nano-particles made of
stacks of graphene layers. The theory also covers cases where graphene is
affected by structural, chemical or orientation disorder, each disorder type
being quantitatively defined by a single parameter. The extinction bumps
carried by such model materials are found to have positions and widths falling
in the same range as the known astronomical 2175 \AA features: as the disorder
parameter increases, the bump width increases from 0.85 to 2.5 $\mu$m$^{-1}$,
while its peak position shifts from 4.65 to 4.75 $\mu$m$^{-1}$. Moderate
degrees of disorder are enough to cover the range of widths of the vast
majority of observed bumps (0.75 to 1.3 $\mu$m$^{-1}$). Higher degrees account
for outliers, also observed in the sky.
  The introduction of structural or chemical disorder amounts to changing the
initial $sp^{2}$ bondings into $sp^{3}$ or $sp^{1}$, so the optical properties
of the model material become similar to those of the more or less amorphous
carbon-rich materials studied in the laboratory: a-C, a-C:H, HAC, ACH, coals
etc. The present treatment thus bridges gaps between physically different model
materials.
