Dark energy is an unknown force driving the accelerated expansion of the universe, according to astrophysics. Discovered in the late 1990s, it began overpowering gravity about nine billion years after the Big Bang. While invisible and not directly measurable, dark energy constitutes approximately 68-70% of the universe, outweighing all visible matter and dark matter. Scientists detected its existence by observing the universe's accelerating expansion, a phenomenon that contradicts earlier assumptions about a decelerating or static universe.