  A Faraday rotation experiment can set limits on the magnetic moment of a
electrically-neutral, dark-matter particle, and the limits increase in
stringency as the candidate-particle mass decreases. Consequently, if we assume
the dark-matter particle to be a thermal relic, our most stringent constraints
emerge at the keV mass scale. We discuss how such an experiment could be
realized and determine the limits on the magnetic moment as a function of mass
which follow given demonstrated experimental capacities.
