The speed of light, denoted as "c," is a fundamental constant in physics that represents the speed at which electromagnetic waves, including visible light, propagate through a vacuum. In the International System of Units (SI), the speed of light is defined as exactly 299,792,458 meters per second (approximately 186,282 miles per second).
It is worth noting that the speed of light can vary depending on the medium through which it travels. For example, light travels slightly slower when passing through materials such as water or glass compared to its speed in a vacuum.
On the other hand, dark energy is a term used in cosmology to describe a hypothetical form of energy that may exist throughout the universe. It is posited as an explanation for the observed accelerated expansion of the universe. Dark energy is thought to exert a repulsive force, counteracting the attractive force of gravity and driving the expansion of the universe to accelerate.
While the nature of dark energy is not yet fully understood, it is currently believed to constitute a significant portion of the total energy density of the universe, accounting for about 68% of the universe's mass-energy content. Its existence and properties are inferred through observations of the universe's large-scale structure, such as the distribution of galaxies and the measurements of the cosmic microwave background radiation.
Despite its name, dark energy is not directly observable, and its exact nature remains one of the major mysteries in modern astrophysics and cosmology. Scientists continue to study and investigate various theoretical models and conduct observational studies to better understand the properties and origin of dark energy.