  It is an appealing possibility that the observed dark matter density in the
universe can be fully explained by SUSY. The current experimental knowledge
indicates that this possibility strongly favors a co-annihilation scenario. In
such scenarios, the mass difference between the next-to-lightest SUSY particle
(the NLSP) and the lightest one (the LSP) is quite small, which assures that
the annihilation cross-section is sufficient not to predict a too large
abundance of dark matter. However, the small mass difference also means that
observing SUSY becomes hard at hadron colliders, where the observation hinges
on the tell-tale signature of missing transverse energy: if the mass difference
NLSP-to-LSP is small, only little energy is carried away by the invisible LSP.
This is also true even if several other SUSY particles are within the kinematic
reach, since these states would to a large extent decay via cascades ending
with an NLSP to LSP decay. A lepton collider does not have this problem. The
clean environment and known initial state at such machines assures that SUSY
can be detected even if the mass difference is very small, provided the
center-of-mass energy is sufficiently high. We present prospects for
observation and precision characterization of SUSY with small mass differences
at the ILC, based on detailed simulations of the ILD detector concept. The
resulting possibility to predict the dark matter relic density is evaluated and
compared to the precision obtained from the Planck mission. Taking a specific
model as an example, we also discuss the synergies from combining ILC and
HL-LHC results.
