  We suggest that supersymmetric dark matter be used to set the energy scale of
a linear $e^+ e^-$ collider. Assuming that the lightest supersymmetric particle
(LSP) is a stable neutralino $\chi$, as in many incarnations of the MSSM with
conserved R parity, previous calculations that include coannihilation effects
have delineated the region of the $(m_{1/2}, m_0)$ plane where the LSP
cosmological relic density lies in the preferred range $0.1 \la \Omega_{\chi}
h^2 \la 0.3$. We evaluate here the total cross section for $e^+ e^- \to$
visible pairs of supersymmetric particles, for different values of $m_{1/2}$
and $m_0$, and investigate how much of the dark matter region can be explored
by $e^+ e^-$ colliders with different centre-of-mass energies $E_{CM}$. We find
that a collider with $E_{CM} = 500$ GeV or 1 TeV can only explore part of the
cosmological region, and that a collider with $E_{CM} = 1.5$ TeV with
sufficient luminosity can explore all of the supersymmetric dark matter region.
