What mysterious force is believed to be causing the universe's expansion to accelerate?

The accelerating expansion of the universe is attributed to dark energy, a mysterious and poorly understood force that makes up about 68% of the total energy density of the universe. Observations, particularly those related to distant supernovae and the cosmic microwave background radiation, indicate that the rate of expansion is not only ongoing but is actually increasing over time. This observed acceleration suggests that a repulsive force, associated with dark energy, is acting counter to the attractive force of gravity.

Dark energy is different from dark matter, which primarily interacts with matter through gravitational forces and is thought to account for the missing mass in galaxies. While both dark energy and dark matter are terms used in modern cosmology, their roles in the universe are distinct; dark matter contributes to the structure and formation of galaxies, while dark energy influences the expansion rate. The notion of anti-gravity isn't a recognized scientific term or concept relevant to the accelerating expansion, and space-time, while fundamental to our understanding of gravity and the universe, does not by itself explain this specific phenomenon. Thus, dark energy is the correct answer, as it is the leading theory explaining the accelerating expansion of the cosmos.