Dark energy is an unexplained force that is causing _____. stars to explode the expansion of the universe to accelerate the universe to contract galaxies to group together

The best choice would be the second statement. Dark energy is an unexplained force that is causing the expansion of the universe to accelerate. It is an unknown form of energy which is the most accepted hypothesis during the 1990s to explain the observations that the universe is expanding at a rate that is accelerating.