The primary driving force behind the expansion of the universe is Dark Energy.

Explanation:

1. Introduction:
The expansion of the universe refers to the phenomenon where the distance between galaxies and other cosmic structures is constantly increasing over time. This observation was first made by Edwin Hubble in the 1920s and has since been confirmed by various astronomical observations.

2. Dark Energy:
Dark Energy is a hypothetical form of energy that is believed to permeate all of space and is responsible for the accelerated expansion of the universe. It is called "dark" because it does not interact with light or other electromagnetic radiation, making it difficult to detect directly.

3. Discovery of Dark Energy:
The existence of Dark Energy was first proposed in the late 1990s when astronomers observed that distant supernovae were fainter than expected, indicating that the expansion of the universe is accelerating. This discovery was awarded the Nobel Prize in Physics in 2011.

4. Role of Dark Energy:
Dark Energy acts as a repulsive force that counteracts the gravitational attraction between galaxies and other cosmic structures. While gravity tends to pull objects together, Dark Energy pushes them apart, causing the expansion of the universe to accelerate over time.

5. Understanding Dark Energy:
Despite its importance, Dark Energy remains poorly understood. It is often associated with the cosmological constant, a term introduced by Albert Einstein in his general theory of relativity. The cosmological constant represents a constant energy density in space, which can drive the accelerated expansion.

6. Proportion of Dark Energy:
Dark Energy is believed to account for about 68% of the total energy content of the universe, with Dark Matter accounting for about 27% and ordinary matter making up the remaining 5%. This means that Dark Energy is the dominant factor influencing the expansion of the universe.

7. Future of the Universe:
The presence of Dark Energy and its acceleration of the universe's expansion have significant implications for the future of the cosmos. It is believed that as the universe continues to expand, galaxies will become more and more separated, eventually leading to a "Big Freeze" scenario where the universe becomes cold and dark.

Conclusion:
In conclusion, Dark Energy is the primary driving force behind the expansion of the universe. Its repulsive nature counteracts gravitational attraction and causes the acceleration observed in the expansion. While Dark Energy remains a mysterious phenomenon, further research and observations are ongoing to better understand its nature and implications for the future of the universe.