What is dark energy and why does it affect the expansion of the universe?

The universe is thought to be made of regular matter, dark matter, and dark energy. Scientists do not know what dark energy is. The two leading theories about dark energy are called the cosmological constant theory and the quintessence theory. Each of these theories explains the expansion of the universe in different ways.

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  Cosmological Constant Theory: This theory suggests that dark energy is the energy associated with the vacuum of space. As such, it would have remained unchanged over the history of the universe. If this theory is correct, dark energy would drive the constant acceleration of the cosmos.

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  Quintessance Theory: In this theory, dark energy would change over time. The acceleration of the cosmos would increase over time potentially causing the universe to eventually tear itself apart. This idea is known as "The Big Rip."

There are other theories that suggest that dark energy doesn't exist at all and cosmic acceleration is related to something happening with gravity.