Measurements of the distances to type Ia supernovae have produced strong evidence that the expansion of the Universe is accelerating, implying the existence of a nearly uniform component of dark energy with negative pressure. We show that constraints to this mysterious component based upon large-scale structure nicely complement the SN Ia data, and that together they require ${\ensuremath{\Omega}}_{X}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}{\ensuremath{\rho}}_{X}/{\ensuremath{\rho}}_{\mathrm{crit}}\ensuremath{\in}(0.6,0.7)$ and ${w\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}p}_{X}/{\ensuremath{\rho}}_{X}l\ensuremath{-}0.6$ ( $95%$ C.L.), for the favored flat Universe. Other cosmological data support this conclusion. The simplest explanation, a cosmological constant, is consistent with this, while some of the other possibilities are not.