Dark energy is a hypothesized form of energy permeating all of space, characterized by an intrinsic negative pressure that drives the accelerated expansion of the universe. Based on observations of distant galaxies and the cosmic microwave background, the existence of this repulsive force is inferred because it counteracts the cumulative gravitational pull of all matter and energy within the cosmos. Unlike matter, which tends to dilute as space expands, dark energy's density appears to remain nearly constant. Its sustained repulsive effect suggests it is a fundamental property of the vacuum of space itself, playing a dominant role in shaping the universe's ultimate fate by steadily increasing the rate of cosmic expansion.