Dark matter may be composed of superWIMPs, superweakly-interacting massive<br />
particles produced in the late decays of other particles. We focus here on the<br />
well-motivated supersymmetric example of gravitino LSPs. Gravitino superWIMPs<br />
share several virtues with the well-known case of neutralino dark matter: they<br />
are present in the same supersymmetric frameworks (supergravity with R-parity<br />
conservation) and naturally have the desired relic density. In contrast to<br />
neutralinos, however, gravitino superWIMPs are impossible to detect by<br />
conventional dark matter searches, may explain an existing discrepancy in Big<br />
Bang nucleosynthesis, predict observable distortions in the cosmic microwave<br />
background, and imply spectacular signals at future particle colliders.