Question

in 1998, scientists discovered that the expansion of the universe has been accelerating. what force is causing this phenomena? dark energy hubbles law the doppler effect background radiation

Explanation:

Dark energy is a theoretical form of energy that exerts a negative, repulsive pressure, behaving in a manner opposite to gravity. It is hypothesized to be responsible for the observed accelerating expansion of the universe. Hubble's law describes the relationship between the distance of galaxies and their recession velocity. The Doppler effect is related to the change in frequency of waves due to relative motion. Background radiation is leftover radiation from the early universe.

Answer:

A. dark energy