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:

Hubble's law describes the universe's expansion but not its acceleration. The Doppler effect explains redshift from moving objects, not cosmic acceleration. Background radiation is leftover heat from the Big Bang, unrelated to accelerated expansion. Dark energy is the hypothesized repulsive force driving the universe's accelerating expansion, discovered in 1998 through observations of distant supernovae.

Answer:

dark energy