  Since the geometry of our universe seems to depend very little on baryonic
matter, we consider a variational principle involving only dark matter and dark
energy which in addition make them depend on each other. There are no
adjustable parameters or scalar fields with appropriate equations of state. No
quintessence. For a pressure-less, three-flat FRW model, the cosmological
"constant" is now a function of time, positive by definition and always small.
Its time derivative or rather its associated parameter w is always negative and
close to minus one. The most interesting point is that the age of the universe
and w itself are correlated. Moreover, this rather unsophisticated model
provides a very limited range for both these quantities and results are in
surprising agreement with observed values. The problem of vacuum energy remains
what it was; the problem of coincidence is significantly less annoying.
