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 the cause of the universe's accelerating expansion. Hubble's law describes the relationship between the distance of galaxies from us and their recessional velocity. The Doppler - effect is related to the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source. Background radiation is the leftover radiation from the early stages of the universe.

Answer:

A. dark energy