Missing,"" ""hidden,"" ""invisible"". . .the adjectives used to describe the universe in some recent books bespeak a concern over a cosmological enigma: is the universe open or closed? If open, then galaxies will continue to recede, stars will bum out, and the lights of heaven will die leaving an infinite desolation. If closed, then there is sufficient matter in space to reverse cosmic expansion and lead to eventual collapse. The deciding variable is the universe's density. As a result of recent findings by X-ray, radio and infrared astronomy, space, it seems, is filled with potent sources of mass hitherto unobserved by those using optical telescopes. Current estimates are that perhaps 90 percent of the universe is such dark matter. Enough to close the universe? That is the question that tantalizes astronomer Disney (University College, Cardiff, Wales). Here, he reviews current methodology in weighing stars and galaxies, indicating the unresolved problems of their clumped distribution in space and many other ponderables. He carefully distinguishes between known dark masses, such as the huge haloes that surround the Milky Way, and hypothetical masses such as the neutrino, formerly assumed to be massless. Disney then goes on to examine the various candidates for what constitute the dark known material, logically rejecting notions that they are dwarf stars or other exotica. Even if we count up all this mysterious, galaxy-related mass and measure it, he contends, it would not be sufficient to close the universe. But it is possible that there is more out there than meets the eye: Disney's arguments are that we may be misled, by what he calls an ""iceberg"" illusion, into underestimating the mass of galaxies. We may even be falsely extending Newton's laws to apply to galactic diameters, he says, by computing mass based on gravitational attraction and galactic velocities. This is all done with considerable verve. When Disney laments the UK's parsimonious approach to astronomical research, you'd like to see some budget-busting just for him.