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.