  We develop an analytical approach for the description of an atomic Bose gas
at unitarity. By focusing in first instance on the evaluation of the
single-particle density matrix, we derive several universal properties of the
unitary Bose gas, such as the chemical potential, the contact, the speed of
sound, the condensate density and the effective interatomic interaction. The
theory is also generalized to describe Bose gases with a finite scattering
length and then reduces to the Bogoliubov theory in the weak-coupling limit.
