The term "dark universe" can refer to two different phenomena: dark matter and dark energy.
Dark Matter: Dark matter is a hypothetical form of matter that does not interact with light or other electromagnetic radiation, making it invisible and "dark" in the sense that it cannot be directly observed. Its presence is inferred from its gravitational effects on visible matter and the structure of the universe. The gravitational pull of dark matter helps to explain the observed motions of galaxies and galaxy clusters. It is estimated that dark matter makes up about 85% of the total matter in the universe, while ordinary matter (the stuff we are made of) accounts for only about 15%. Despite its invisible nature, dark matter plays a crucial role in the formation and evolution of structures in the universe.
Dark Energy: Dark energy is another mysterious component of the universe. It is a hypothetical form of energy that is believed to be responsible for the observed accelerating expansion of the universe. Unlike dark matter, dark energy is not associated with the presence of matter. Instead, it is thought to be a property of space itself. Dark energy has a repulsive effect, causing galaxies and other cosmic structures to move away from each other at an accelerating rate. The exact nature of dark energy is still not well understood, and it constitutes the majority of the energy content of the universe.
So, when we talk about the "dark universe," we are referring to the fact that the majority of the matter and energy in the universe is in the form of dark matter and dark energy, which cannot be directly detected or observed using traditional methods of astronomy.