Invisible Matter and the Fate of the Universe

Astronomers and cosmologists believe that as much as 99% of the matter in our universe is hidden from our view. Astronomer Parker engagingly details the ongoing search for this ``missing matter'' and its implications for the ultimate fate of our universe: if there is enough matter, then the universe is ``closed'' and gravity will cause it to collapse into the ``big crunch''; if visible matter is all there is, the universe will expand forever and gradually stars, galaxies and even protons will evaporate. What the dark matter might consist of is debated. Candidates include a variety of exotic particles (axions, photinos), magnetic monopoles, massive neutrinos, or faint, undernourished stars called brown dwarfs. However, there may be no ``missing matter,'' we also learn in this accessible guide to a scientific controversy. Illustrations not seen by PW. (Oct.)