  We consider the universe as a huge $\nu_R$-sphere formed with degenerate
relic neutrinos and suggest that its constant energy density play a role of an
effective cosmological constant. We construct the sphere as a bubble of true
vacuum in a field theory model with a spontaneously broken U(1) global
symmetry, and we interpret the sphere-forming time as the transition time for
recent acceleration of the universe. The coincidence problem may be regarded as
naturally resolved in this model, because the relic neutrinos can make the
$\nu_R$-sphere at the recent past time during the matter-dominated era.
