  Lack of knowledge about the detailed many-particle motion on the microscopic
scale is a key issue in any theoretical description of a macroscopic
experiment. For systems at or close to thermal equilibrium, statistical
mechanics provides a very successful general framework to cope with this
problem. Far from equilibrium, only very few quantitative and comparably
universal results are known. Here, a new quantum mechanical prediction of this
type is derived and verified against various experimental and numerical data
from the literature. It quantitatively describes the entire temporal relaxation
towards thermal equilibrium for a large class (in a mathematically precisely
defined sense) of closed many-body systems, whose initial state may be
arbitrarily far from equilibrium.
