  The evidence is that the mass of the universe is dominated by an exotic
nonbaryonic form of matter largely draped around the galaxies. It approximates
an initially low pressure gas of particles that interact only with gravity, but
we know little more than that. Searches for detection thus must follow many
difficult paths to a great discovery, what the universe is made of. The
nonbaryonic picture grew out of a convergence of evidence and ideas in the
early 1980s. Developments two decades later considerably improved the evidence,
and advances since then have made the case for nonbaryonic dark matter
compelling.
