The current understanding of the expansion of the universe is based on observations and theoretical models, but there are still some aspects that remain uncertain. According to the prevailing model, the universe has been experiencing accelerated expansion for the past several billion years. This acceleration is attributed to a component called dark energy, which is believed to be responsible for the observed phenomenon.
Before the discovery of dark energy, the prevailing belief was that the expansion of the universe should be slowing down over time due to the gravitational pull of matter. This was based on the understanding that matter exerts an attractive force on other matter through gravity. The more matter present in the universe, the stronger the gravitational force, and hence the slower the expansion.
However, in the late 1990s, observations of distant supernovae and other cosmological measurements indicated that the expansion of the universe was actually accelerating, contrary to the previous expectations. This discovery led to the hypothesis that a new form of energy, referred to as dark energy, permeates the universe and is responsible for driving this acceleration.
The nature of dark energy is still not well understood. It is often associated with a cosmological constant or a scalar field called quintessence. Dark energy, unlike matter, has a repulsive effect on the expansion of the universe, causing it to accelerate.
As for the question of whether there was no dark energy before a certain point in time, it is challenging to make definitive statements without more information. The current understanding is that dark energy has existed throughout the history of the universe, but its influence on the expansion rate may have been negligible during certain periods. The exact mechanisms and properties of dark energy are still the subject of ongoing research and exploration in cosmology.