The mystery of the dark matter may be the harbinger of another scientific revolution."" So say the Tuckers (he: an X-ray astonomer/physicist, UC at Irvine; she: his collaborator on several good popularizations, e.g., The Cosmic Inquirers, 1986). The dark matter made its debut in the 1930's with the discoveries of Jan Oort (for whom the cloud of comets at the outer edge of the solar system is named). Oort established that the Milky Way rotates around the galactic center and that stars in our region bob up and down under the force of gravity. But the mass required to account for the star motions is far in excess of all known masses: stars, dust, comets, etc. The Space Age opened up new possibilities for the dark matter: radio and X-ray astronomy revealed exotic neutron stars, pulsars, quasars, black holes--but still not enough to account for projected ranges of 50 to 90 percent for the amount of dark matter in the universe. ""Brown"" dwarfs--feeble little stars--are a possibility, but more intriguing are the huge dark envelopes that appear to surround spiral galaxies like the Milky Way and even large clusters of galaxies. What inhabits these clouds? ""Cosmions""--also called WIMPS (weakly interacting massive particles) have been proposed. These supposedly are leftovers from the Big Bang. They might account for envelope matter, but still leaves Other dark matter, in the central disc of the galaxy, for example, up for grabs. The Tuckers make it clear that speculation and circumstantial evidence abound and are leading to visions and revisions of what seemed conventional wisdom only yesterday: the Big Bang, the Inflationary Universe, the notion of gravity. And because they have to review the consensus in order to explain the crisis, the reader is treated to an up-to-date summation of cosmology, along with some astute comments on the makers and shakers, including the position of women in the field.