  Friedmann-Robertson-Walker universes with a presently large fraction of the
energy density stored in an $X$-component with $w_X<-1/3$, are considered. We
find all the critical points of the system for constant equations of state in
that range. We consider further several background quantities that can
distinguish the models with different $w_X$ values. Using a simple toy model
with a varying equation of state, we show that even a large variation of $w_X$
at small redshifts is very difficult to observe with $d_L(z)$ measurements up
to $z\sim 1$. Therefore, it will require accurate measurements in the range
$1<z<2$ and independent accurate knowledge of $\Omega_{m,0}$ (and/or
$\Omega_{X,0}$) in order to resolve a variable $w_X$ from a constant $w_X$.
