  Models with late time cosmic acceleration, such as the Lambda-dominated CDM
model, predict a freeze out for the growth of linear gravitational potential at
moderate redshift z<1, what can be observed as temperature anisotropies in the
CMB: the so called integrated Sachs-Wolfe (ISW) effect. We present a direct
measurement of the ISW effect based on the angular cross-correlation function,
w_{TG}, of CMB temperature anisotropies and dark-matter fluctuations traced by
galaxies. We cross-correlate the first-year WMAP data in combination with the
APM Galaxy survey. On the largest scales, theta = 4-10 deg, we detect a
non-vanishing cross-correlation at 98.8 % significance level, with a 1-sigma
error of w_{TG} = 0.35 +/- 0.14 microK, which favors large values of
Omega_Lambda \simeq 0.8 for flat FRW models. On smaller scales, theta < 1deg,
the correlations disappear. This is contrary to what would be expected from the
ISW effect, but the absence of correlations may be simply explained if the ISW
signal was being cancelled by anti-correlations arising from the thermal
Sunyaev-Zeldovich (SZ) effect.
