  We conducted micro-gravity experiments to study the outcome of collisions
between sub-mm-sized dust agglomerates consisting of \mu m-sized SiO2 monomer
grains at velocities of several cm/s. Prior to the experiments, we used X-ray
computer tomography (nano-CT) imaging to study the internal structure of these
dust agglomerates and found no rim compaction so that their collision behavior
is not governed by preparation-caused artefacts. We found that collisions
between these dust aggregates can lead either to sticking or to bouncing,
depending mostly on the impact velocity. While previous collision models
derived the transition between both regimes from contact physics, we used the
available empirical data from these and earlier experiments to derive a power
law relation between dust-aggregate mass and impact velocity for the threshold
between the two collision outcomes. In agreement with earlier experiments, we
show that the transition between both regimes is not sharp, but follows a
shallower power law than predicted by previous models (G\"uttler et al. 2010).
Furthermore, we find that sticking between dust aggregates can lead to the
formation of larger structures. Collisions between aggregates-of-aggregates can
lead to growth at higher velocities than homogeneous dust agglomerates.
