Abstract: The nature of the cosmic dark matter is one of the great mysteries in physics today. The best known candidate is the neutralino, the lightest superpartner in supersymmetric extensions of the Standard Model. We investigate the ability of the Large Hadron Collider (LHC) and the proposed International Linear Collider (ILC) to measure the properties of this dark matter candidate. Of interest to astrophysics are the relic density, direct detection cross sections, annihilation cross sections, and annihilation branching ratios to the gamma ray lines. In a general 24 parameter supersymmetric model, we illustrate how well these quantities are constrained by the LHC and ILC for several benchmark supersymmetric scenarios. Furthermore, we illustrate how astrophysical measurements of these quantities contribute to collider studies of the supersymmetric particle spectrum.