  Unless properly corrected for, the existence of intergalactic dust will
introduce a redshift dependent magnitude offset to standard candle sources.
This would lead to overestimated luminosity distances compared to a dust-free
universe and bias the cosmological parameter estimation as derived from, e.g.,
Type Ia supernovae observations. In this paper, we model the optical extinction
and X-ray scattering properties of intergalactic dust grains to constrain the
intergalactic opacity using a combined analysis of observed quasar colours and
the soft X-ray background. Quasar colours effectively constrain the amount of
intergalactic dust grains smaller than ~0.2 microns, to the point where we
expect the corresponding systematic error in the Type Ia supernova
magnitude-redshift relation to be sub-dominant. Soft X-ray background
observations are helpful in improving the constraints on very large dust grains
for which the amount of optical reddening is very small and therefore is more
difficult to correct for. Our current upper limit corresponds to ~0.25
magnitude dimming at optical wavelengths for a source at redshift z = 1, which
is too small to alleviate the need for dark energy but large in terms of
relative error. However, we expect it to be possible to lower this bound
considerably with an improved understanding of the possible sources of the
X-ray background, in combination with observations of compact X-ray sources
such as Active Galactic Nuclei.
