People lose their car keys and insurance cards, and any one of a pair of socks can vanish as surely as if it had been sucked into another dimension. But lose most of the universe? Now there's a feat--and one that has been accomplished. Preposterous as this sounds, astronomers and cosmologists are in wide agreement that most of the universe is missing. The latest physics may suggest where the missing parts of the universe are and in turn provide some hints about much larger questions--including whether there is an ethereal reality all around us.
Here's why scientists think most of the cosmos is missing: The heavens behave as though being acted on by far more material than meets the eye. For instance, when cosmologists "weigh" the universe by estimating the mass of the components that shine, and thus are easy to detect (mainly, stars and nebulae), it appears the galaxies don't contain anywhere near the amount of matter necessary to keep them from flying apart. Galaxies spin like disks, and stars on the outer rim of our Milky Way are whirling with so much speed and momentum, they ought to overcome the galaxy's gravity and soar off into the interstellar void. Yet they don't; the heavens remain ordered. This causes cosmologists to assume that there must be an enormous amount of "missing mass," undetected material that keeps the universe stable. Current estimates hold that in order for the universe to behave the way it does, visible matter can make up only 10% to a third of the firmament. The rest of the universe, two-thirds to as much as 90%, is "missing."
Where or what is the missing mass? It's probably not in black holes. For techno-reasons that can be skipped here, even though black holes are the most massive objects that can be imagined, they probably do not account for the missing substance. The leading theory is, instead, that the universe is rife with an unknown "dark matter" or even "dark energy." Not dark as in the Dark Side--"dark" in this usage means "We don't see it." But there could well be a mystical implication, which we'll return to in a moment.
What might dark matter be? The first credible theory was "brown dwarfs." This idea held that each galaxy contains many billions of objects similar to the planet Jupiter--very large by the standard of planets, but not large enough to ignite into stars. If brown dwarfs were everywhere, astronomers would have a hard time finding them because these objects would not shine, but they might account for the missing mass. Recently, the brown-dwarf theory has fallen on hard times, mainly because the material for brown dwarfs would have to come from supernovae, and there don't seem to have been enough of these explosions in cosmic history to generate the required ingredients.
A few years ago, a new theory arose: The missing mass was in "machos," short for Massive Compact Halo Objects. This theory held that each galaxy is surrounded by a haze of old, collapsed stars that are very dense and hold lots of matter, but hardly shine; their gravity orders the heavens. Macho theory has been in decline lately, however, despite its wonderful name. If there were vast numbers of old macho stars, they should have filled the galaxies with infrared radiation in the act of becoming old, and that effect is not observed.
Next come the weird-matter theories, currently ascendant among scientists. Playing off the name machos, researchers have supposed that the missing matter will be found in the equally wonderfully named "wimps," short for Weakly Interacting Massive Particles. (Yes, these two names result in endless science-journal headlines along the lines of, "Machos Beat Up on Wimps.")
If they exist, wimps would be subatomic particles fundamentally different from the stuff that you and I and the rocks and hills and vales are made of, which physicists call "baryonic" matter. (Baryonic matter is the protons and neutrons you learned about in high school; barus means "heavy" in Greek.) Wimps would contain huge amounts of mass and thus supply the missing gravity that keeps galaxies stable, yet otherwise would have almost no interactions with the standard form of matter of which we and the stars are made. Wimps would have no positive charge like a proton, or negative charge like an electron, and would lack other features of standard subatomic particles.
In theory, the universe could contain unimaginable amounts of wimps, and we wouldn't realize it because the wimps would be passing through us without interacting with the stuff we are made out of. There might also be ghostly "dark energy" that is ubiquitous but undetected because it does not interact with the standard energy we know of. (Since, by Einstein's dictum, matter and energy are equivalent, dark energy might solve the missing-mass problem even though it isn't mass--but we'll skip the reasoning on this point.)
Are wimps everywhere? In February, a group of physicists at the Gran Sasso National Laboratory in Italy reported an experimental finding suggesting that apparently empty deep space is actually a boundless mist of wimps through which the galaxies travel. Other researchers, especially from the Cold Dark Matter Search project at Stanford University, have questioned many details of the Italian finding.