  Supersymmetry can explain the observed dark matter relic density with a
neutralino dark matter particle and a coannihilating, almost mass-degenerate
sparticle. If this were the case in nature, a linear electron positron collider
like the ILC could discover the two sparticles if their masses are in the
kinematic reach of the collider. This contribution discusses which observations
are necessary at the ILC for predicting the dark matter relic density correctly
and for confirming that the observed lightest neutralino is the only kind of
dark matter. We take the case of stau coannihilation as an example.
