Dark Dust and single-cloud sightlines in the ISM
  The precise characteristics of clouds and the nature of dust in the diffuse
interstellar medium can only be extracted by inspecting the rare cases of
single-cloud sightlines. In our nomenclature such objects are identified by
interstellar lines, such as K I, that show at a resolving power of $\lambda
/\Delta \lambda \sim 75,000$ one dominating Doppler component that accounts for
more than half of the observed column density. We searched for such sightlines
using high-resolution spectroscopy towards reddened OB stars for which far-UV
extinction curves are known. We compiled a sample of 186 spectra, 100 of which
were obtained specifically for this project with UVES. In our sample we
identified 65 single-cloud sightlines, about half of which were previously
unknown. We used the CH/CH$^+$ line ratio of our targets to establish whether
the sightlines are dominated by warm or cold clouds. We found that CN is
detected in all cold (CH/CH$^+ >1$) clouds, but is frequently absent in warm
clouds. We inspected the WISE ($3-22\, \mu$m) observed emission morphology
around our sightlines and excluded a circumstellar nature for the observed dust
extinction. We found that most sightlines are dominated by cold clouds that are
located far away from the heating source. For 132 stars, we derived the
spectral type and the associated spectral type-luminosity distance. We also
applied the interstellar Ca II distance scale, and compared these two distance
estimates with GAIA parallaxes. These distance estimates scatter by 40%. By
comparing spectral type-luminosity distances with those of GAIA, we detected a
hidden dust component that amounts to a few mag of extinction for eight
sightlines. This Dark Dust is populated by $\ge 1 \mu$m large grains and
predominately appears in the field of the cold interstellar medium.
