Clues about the nature of the missing remainder emerge from elaborate computer simulations showing how large-scale cosmic structures—clusters and superclusters of galaxies—evolved out of a slight lumpiness in the early universe. Over time, gravity amplified the lumpiness by causing the denser regions to collapse and fragment. The manner in which that happened depends on the properties of the predominant form of matter. Ordinary atoms are easily agitated by radiation, so they would take too long to settle into the observed structures. The simulations reveal the likely nature of the matter that seeded the formation of today’s galaxy clusters. It should be dark—that is, it shouldn’t get stirred up by radiation—and it should be slow moving relative to the speed of light, or “cold” in astronomical parlance. (Neutrinos, in contrast, are fast moving, or “hot.”) Literal-minded cosmologists have named this stuff cold dark matter.

The latest theories that attempt to construct a unified model of physics list a number of potential particles that seem to match these properties. The exotic particles, the lightest of which would be at least 100 times as heavy as a hydrogen atom, are referred to as weakly interacting massive particles, or WIMPs. They are purely hypothetical, but if they exist it should be possible to detect them. If there are enough WIMPs to make up all the dark matter in our galaxy, then there should be several thousand per cubic meter in the region around Earth. They would be moving at about the same speed as the average star in our galaxy, 135 miles per second. Most of these electrically neutral particles would, like neutrinos, go straight through Earth. On rare occasions, however, one might interact with an atom in the material they pass through.

Several international collaborations, including the DAMA (DArk MAtter) project at the Gran Sasso laboratory in Italy and the UK Dark Matter Collaboration headquartered at the Boulby mine in England, have designed experiments to detect the minuscule recoil when a WIMP hits a slab of ultrapure silicon or some equivalent material. These detectors must be cooled to an extremely low temperature and placed deep underground to eliminate other kinds of particle impacts that could drown out the dark matter signal. So far, the only claimed detection of a dark matter particle—made three years ago by a team at the University of Rome—has been strongly disputed. Meanwhile, the search goes on at ever-increasing sensitivity.


What Is the Unseen Cosmos Made Of?

Dark energy may arise from the physical properties of empty space, similar to the cosmological constant, a long-range repulsion proposed by Albert Einstein in 1917. Or it may be a novel type of field, called quintessence, that arose alongside the various types of matter when the universe was born.

Dark matter most likely includes several different kinds of particles. One group consists of relatively fast-moving, or “hot,” particles. These are probably neutrinos, ghostly particles that were discovered decades ago but only recently found to have a small mass. Another group consists of more sluggish, or “cold,” particles. These may be WIMPs, heavyweight counterparts to known particles, or another, less massive class of particles called axions. But nobody has ever detected a WIMP or an axion.

Ordinary hidden matter consists of atoms that emit little or no light. Planets are a form of nonluminous matter. Gas clouds can be too. Brown dwarfs, less massive than stars, are nearly dark, as are collapsed stars—white dwarfs, neutron stars, and black holes. Even cometlike clumps of frozen hydrogen could explain some hidden matter, although nobody knows how such chunks could form.           

 —C.S.P.