Abstract : We report the detection of the Integrated Sachs-Wolfe effect (ISW) at scales in the sky around [5, 10] degrees with a significance > 3.7 sigma, by cross-correlating the Wilkinson Anisotropy Probe (WMAP) first-year data and the NRAO VLA Sky Survey (NVSS) in the Spherical Mexican Hat Wavelet (SMHW) space. This result represents the highest signifcance level of the ISW detection (also of the dark energy, assuming a flat universe) reported up to date. We also show that the cross-correlation signal is not caused neither by systematic effects nor foreground contamination. In addition, we can put strong constraints for the amount of the dark energy and the equation of the state parameter (w), by comparing the measured ISW signal with the one predicted by different cosmological models. For a bias value of b = 1.6, we obtain 0.61 <= dark energy density = 0.85 and w <= -0.81 (at 2 sigma CL). These results do not change very significantly for other realistic values of the bias parameter. The dark energy is clearly detected since the dark eneryg density > 0 at > 5 sigma CL. This is the first estimation of the equation of state of the dark energy made through the cross-correlation of the CMB and the nearby galaxy density distribution. It also provides an independent estimation from the one made by the WMAP team using CMB and LSS.