  Dust emission at sub-millimetre wavelengths allows us to trace the early
phases of star formation in the Universe. In order to understand the physical
processes involved in this mode of star formation, it is essential to gain
knowledge about the dark matter structures - most importantly their masses -
that sub-millimetre galaxies live in. Here we use the magnification effect of
gravitational lensing to determine the average mass and dust content of
sub-millimetre galaxies with 250mu flux densities of S_250>15mJy selected using
data from the Herschel Multi-tiered Extragalactic Survey. The positions of
hundreds of sub-millimetre foreground lenses are cross-correlated with the
positions of background Lyman-break galaxies at z~3-5 selected using optical
data from the Canada-France Hawaii Telescope Legacy Survey. We detect a
cross-correlation signal at the 7-sigma level over a sky area of one square
degree, with ~80% of this signal being due to magnification, whereas the
remaining ~20% comes from dust extinction. Adopting some simple assumptions for
the dark matter and dust profiles and the redshift distribution enables us to
estimate the average mass of the halos hosting the sub-millimetre galaxies to
be log(M_200/M_sun)=13.17+0.05-0.08(stat.) and their average dust mass fraction
(at radii of >10kpc) to be M_dust/M_200~6x10^-5. This supports the picture that
sub-millimetre galaxies are dusty, forming stars at a high rate, reside in
massive group-sized halos, and are a crucial phase in the assembly and
evolution of structure in the Universe.
