4.4. Dark Matter: What, and How Much?

The nature of the dark matter - how much there is and what it is - still eludes us. It's embarrassing that 90 percent of the universe remains unaccounted for.

This key question may yield to a three-pronged attack:

1. Direct detection. Astronomical searches are underway for ``machos'' in the Galactic Halo; and several groups are developing cryogenic detectors for supersymmetric particles and axions.

2. Progress in particle physics. Important recent measurements suggest that neutrinos have non-zero masses; this result has crucially important implications for physics beyond the standard model; however the inferred masses seem too low to be cosmologically important. If theorists could pin down the properties of supersymmetric particles, the number of particles that survive from the big bang could be calculated just as we now calculate the helium and deuterium made in the first three minutes. Optimists may hope for progress on still more exotic options.

3. Simulations of galaxy formation and large-scale structure. When and how galaxies form, the way they are clustered, and the density profiles within individual systems, depend on what their gravitationally-dominant constituent is, and are now severely constraining the options.