  The dark-matter candidates of particle physics invariably possess
electromagnetic interactions, if only via quantum fluctuations. Taken en masse,
dark matter can thus engender an index of refraction which deviates from its
vacuum value. Its presence is signaled through frequency-dependent effects in
the propagation and attenuation of light. We discuss theoretical constraints on
the expansion of the index of refraction with frequency, the physical
interpretation of the terms, and the particular observations needed to isolate
its coefficients. This, with the advent of new opportunities to view gamma-ray
bursts at cosmological distance scales, gives us a new probe of dark matter and
a new possibility for its direct detection. As a first application we use the
time delay determined from radio afterglow observations of distant gamma-ray
bursts to realize a direct limit on the electric-charge-to-mass ratio of dark
matter of |varepsilon|/M < 1 x 10^{-5} eV^{-1} at 95% CL.
