  The first-year WMAP data, in combination with any one of a number of other
cosmic probes, show that we live in a flat \Lambda-dominated CDM universe with
\Omega_m ~ 0.27 and \Omega_\Lambda ~ 0.73. In this model the late-time action
of the dark energy, through the integrated Sachs-Wolfe effect, should produce
CMB anisotropies correlated with matter density fluctuations at z<2 (Crittenden
& Turok 1996). The measurement of such a signal is an important independent
check of the model. We cross-correlate the NRAO VLA Sky Survey radio source
catalog (Condon et al. 1998) with the WMAP data in search of this signal, and
see indications of the expected correlation. Assuming a flat \Lambda-CDM
cosmology, we find \Omega_\Lambda>0 (95% CL, statistical errors only) with the
peak of the likelihood at \Omega_\Lambda=0.68, consistent with the preferred
WMAP value. A closed model with \Omega_m=1.28, h=0.33, and no dark energy
component (\Omega_\Lambda=0), marginally consistent with the WMAP CMB TT
angular power spectrum, would produce an anti-correlation between the matter
distribution and the CMB. Our analysis of the cross-correlation of the WMAP
data with the NVSS catalog rejects this cosmology at the 3\sigma level.
