Final answer:
Dark energy is unexpectedly causing the universe's expansion to accelerate, counteracting gravitational forces that were once slowing it down.
Step-by-step explanation:
The impact of dark energy on the expansion of the universe is significant and somewhat counterintuitive. Initially, the expansion of the universe was most strongly influenced by gravity, causing it to slow down as matter was closer together. However, as the universe has grown, dark energy, which acts over large scales, has become more important. This mysterious form of energy is causing the rate of expansion to accelerate, leading to an ever-faster expanding universe. This was an unexpected discovery, and it contrasts with earlier beliefs that the universe's expansion would be slowing down due to gravitational attraction between galaxies.
The discovery that the universe is not only expanding but accelerating in its expansion was shocking to scientists, as it suggested the presence of an unknown form of energy supplying this acceleration. This has led to the era known as the modern inflationary era, which is characterized by the acceleration of the universe's expansion. Therefore, the correct impact of dark energy is that it is making the expansion faster which was unexpected.