  Distance estimates derived from spectroscopy or parallax have been unified by
considering extinction by large grains. The addition of such a population of
what is called Dark Dust to models of the diffuse interstellar medium is tested
against a contemporary set of observational constraints. The dark dust model
explains, by respecting representative solid-phase element abundances,
simultaneously the typical wavelength-dependent reddening, extinction, and
emission of polarized and unpolarized light by interstellar dust particles
between far UV and millimetre wavelengths. The physical properties of dark dust
are derived. Dark dust consists of micrometre-sized particles, which have been
recently detected in-situ. It provides significant wavelength-independent
reddening from the far UV to the near-infrared. Light absorbed by dark dust is
re-emitted in the submillimeter region by grains at dust temperatures of 8-12K.
Such very cold dust has been frequently observed in external galaxies. Dark
dust contributes to the polarisation at greater than about 1 mm to ~35% and at
shorter wavelengths marginally. Optical constants for silicate dust analogous
are investigated. By mixing 3% in mass of Mg$_{0.8}$Fe$^{2+}_{0.2}$ SiO$_3$ to
MgO$-$0.5 SiO$_2$ a good fit to the data is derived that still can accommodate
up to 5 - 10% of mass in dark dust. The extra diming of light by dark dust is
unexplored when discussing SN~Ia light curves and in other research. Previous
models that ignore dark dust do not account for the unification of the distance
scales.
