What evidence supports the existence of dark energy as the dominant force in the universe?

The evidence supporting the existence of dark energy as the dominant force in the universe primarily arises from measurements of the accelerating expansion of the universe and the geometry of the cosmos. In the late 1990s, observations of distant supernovae revealed that the rate at which the universe is expanding is not slowing down, as was previously expected, but is instead accelerating. This acceleration suggests that there is a force acting in opposition to gravity on cosmological scales.

The geometry of the universe can also provide insight into dark energy. Observations, particularly from the Cosmic Microwave Background (CMB) and the distribution of galaxies, indicate that the universe is flat, which implies a certain energy density. Combining measurements of the rate of expansion with the density parameters of the universe allows astronomers to conclude that a significant portion of the universe's energy density must come from a mysterious form of energy – now referred to as dark energy.

This understanding resolves several outstanding questions about the behavior of the universe on large scales and aligns with the predictions made by the theory of General Relativity. Thus, the detailed observations of the universe's accelerated expansion and geometry are strong indicators of the presence and dominance of dark energy.