The phenomenon you're referring to is called the "accelerating expansion of the universe," and it is a topic of great interest and ongoing research in cosmology. The observed acceleration of the universe's expansion is attributed to a concept known as dark energy.
Dark energy is a hypothetical form of energy that is thought to permeate all of space and generate a repulsive gravitational effect. It is often associated with the cosmological constant, which was originally introduced by Albert Einstein in his theory of general relativity. The cosmological constant represents a constant energy density in space, which can cause a repulsive gravitational force.
The acceleration of the universe's expansion can be explained by the interplay between gravity, matter (including both normal matter and dark matter), and dark energy. On large scales, gravity is responsible for the attractive force between matter and acts to slow down the expansion of the universe. However, as the universe continues to expand, the density of matter decreases, and the influence of dark energy becomes relatively more significant.
Dark energy, with its negative pressure, exerts a repulsive gravitational effect that counteracts the attractive force of gravity on cosmic scales. As the universe expands, the amount of dark energy per unit volume remains constant, leading to an increasing dominance of dark energy over matter. This results in an acceleration of the universe's expansion, even in the absence of additional mass or matter.
The exact nature of dark energy and the mechanism behind its existence remain poorly understood. It is still an active area of research, and scientists are exploring different theories and observational methods to gain a deeper understanding of this phenomenon. Some proposed explanations for dark energy include the cosmological constant, quintessence (a dynamic field that evolves with time), or modifications to general relativity.
It's important to note that while the accelerating expansion of the universe is a widely accepted observational result, the precise nature of dark energy and the underlying physics involved are still subjects of investigation and debate among scientists.