  The incompressible Navier-Stokes equations coupled to the Maxwell-Stefan
relations for the molar fluxes are analyzed in bounded domains with no-flux
boundary conditions. The system models the dynamics of a multicomponent gaseous
mixture under isothermal conditions. The global-in-time existence of bounded
weak solutions to the strongly coupled model and their exponential decay to the
homogeneous steady state are proved. The mathematical difficulties are due to
the singular Maxwell-Stefan diffusion matrix, the cross-diffusion terms, and
the Navier-Stokes coupling. The key idea of the proof is the use of a new
entropy functional and entropy variables, which allows for a proof of positive
lower and upper bounds of the mass densities without the use of a maximum
principle.
