  We present cosmological parameter constraints on flat cosmologies dominated
by dark energy using various cosmological data including the recent Archeops
angular power spectrum measurements. A likelihood analysis of the existing
Cosmic Microwave Background data shows that the presence of dark energy is not
requested, in the absence of further prior. This comes from the fact that there
exist degeneracies among the various cosmological parameters constrained by the
Cosmic Microwave Background. We found that there is a degeneracy in a
combination of the Hubble parameter H_0 and of the dark energy equation of
state parameter w_Q, but that w_Q is not correlated with the primordial index n
of scalar fluctuations and the baryon content Omega_b h^2. Preferred primordial
index is n = 0.95 \pm 0.05 (68%) and baryon content Omega_b h^2 = 0.021 \pm
0.003. Adding constraint on the amplitude of matter fluctuations on small
scales, sigma_8, obtained from clusters abundance or weak lensing data may
allow to break the degenaracies, although present-day systematics uncertainties
do not allow firm conclusions yet. The further addition of the Hubble Space
Telescope measurements of the local distance scale and of the high redshift
supernovae data allows to obtain tight constraints. When these constraints are
combined together we find that the amount of dark energy is 0.7^{+0.10}_{-0.07}
(95% C.L.) and that its equation of state is very close to those of the vacuum:
w_Q < -0.75 (> 95% C.L.). In no case do we find that quintessence is prefered
over the classical cosmological constant, although robust data on sigma_8 might
rapidly bring light on this important issue.
