The origin of dark matter in the Universe may be scalar particles produced
by amplification of quantum fluctuations during a period of dilaton-driven
inflation. We show, for the first time, that a single species of such parti
cles, depending on its mass and interactions, can be a source of both cold
and hot dark matter simultaneously. Detection of such weakly interacting pa
rticles with masses below a fraction of an eV presents a new challenge for
dark matter searches. [S0031-9007(99)08891-2].