We present measurements of the angular cross-correlation between luminous red galaxies from the Sloan Digital Sky Survey and the cosmic microwave background temperature maps from the Wilkinson Microwave Anisotropy Probe. Looking at a number of redshift slices and CMB bands, we find a statistically significant achromatic positive correlation between these data sets, consistent with the expected signal from the late Integrated Sachs-Wolfe effect. We do not detect any anti-correlation on small angular scales as would be produced by a large Sunyaev-Zel’dovich effect, although we do see evidence for some SZ effect in our highest redshift samples. Assuming the flat universe found by the WMAP survey, our preliminary detection of the ISW effect provides independent physical evidence for the existence of dark energy.