Dark energy is a theoretical form of energy that is hypothesized to exist in order to explain the observed accelerated expansion of the universe. It is believed to constitute a significant portion of the total energy density of the universe, though its precise nature and origin are still not well understood.
The concept of dark energy emerged from observations in the late 1990s that showed the expansion of the universe is actually accelerating, contrary to what was expected based on the gravitational attraction of matter. Dark energy is postulated as a possible explanation for this accelerated expansion.
The specific mechanism by which dark energy causes the accelerated expansion is not fully understood. However, it is often associated with a property called "negative pressure." In the framework of general relativity, the gravitational interaction between matter and energy determines the curvature of spacetime. Matter and ordinary forms of energy exert positive pressure, tending to slow down the expansion of the universe. In contrast, dark energy is thought to possess negative pressure, exerting a repulsive gravitational effect, which drives the expansion of space to accelerate.
This accelerated expansion does not violate the principle that nothing can travel faster than the speed of light. It is important to note that the expansion of the universe is not a motion of objects through space but rather a stretching of the fabric of spacetime itself. As the universe expands, it carries galaxies and other objects along with it. The expansion of space between these objects can cause them to move apart at speeds that are greater than the speed of light, but this does not contradict the fundamental limit on the speed of light within spacetime.
However, it is worth mentioning that the accelerated expansion of the universe does not directly cause all matter to move faster than light. Within local systems, such as galaxies or galaxy clusters, gravitational forces dominate over the expansion of the universe. These local systems can still have relative velocities that are much smaller than the speed of light. The accelerated expansion affects the large-scale structure of the universe, leading to the separation of clusters and galaxies on cosmological scales.