Dark energy is a property of empty space. It has a repulsive effect on matter. As the universe expands there is more empty space therefore more dark energy. More dark energy = more repulsive force on the matter of the universe. This is why we see an accelerating universe. But it wasn't always accelerating. In the early universe there was less empty space and therefore less dark energy and less repulsive force. At an early time the attractive forces of matter and dark matter predominated over the repulsive force of dark energy and the universe was decelerating. At some point though the universe gained a critical amount of empty space and the repulsive affect of dark energy began to dominate. The transition from deceleration to acceleration is termed a "jerk". There was a cosmic jerk.