If gamma-ray bursters are at cosmological distances, they can be used to probe for "femtolenses," extremely small (~10^-13^-10^-16^M_sun_) dark-matter objects. The time delay induced by such a lens would be ~10^-17^-10^-20^ s, approximately equal to the period of a gamma ray. Interference between the two images would induce a characteristic interference pattern as a function of detected frequency. The pattern would be stable on time scales of 1 s, but might slowly drift on time scales of 10 s in response to the relative motion of the lens and source. The signal from a femtolens would be quite distinct and would be well within the sensitivity of present detectors.
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Andrew Gould (1992) studied this question.