  The Minimal Supersymmetric Standard Model opens the possibility of
electroweak baryogenesis provided that the light scalar top quark (stop) is
lighter than the top quark. In addition, the lightest neutralino is an ideal
candidate to explain the existence of dark matter. For a light stop with mass
close to the lightest neutralino, the stop-neutralino co-annihilation mechanism
becomes efficient, thus rendering the predicted dark matter density compatible
with observations. Such a stop may however remain elusive at hadron colliders.
Here it is shown that a future linear collider provides a unique opportunity to
detect and study the light stop. The production of stops with small
stop-neutralino mass differences is studied in a detailed experimental analysis
with a realistic detector simulation including a CCD vertex detector for flavor
tagging. Furthermore, the linear collider, by precision measurements of
superpartner masses and mixing angles, also allows to determine the dark matter
relic density with an accuracy comparable to recent astrophysical observations.
