What is dark energy believed to be responsible for?

Dark energy is a mysterious force that is thought to make up about 68% of the universe and is believed to be responsible for the accelerated expansion of the universe. This expansion was first observed in the late 1990s when astronomers discovered that distant supernovae were dimmer than expected, suggesting that the universe was not only expanding but doing so at an increasing rate.

The concept of dark energy helps explain why the expansion of the universe is accelerating rather than slowing down due to the gravitational attraction of matter. As the universe expands, dark energy appears to exert a repulsive force that accelerates this expansion. This phenomenon is critical in cosmology, as it challenges previous models of the universe and influences our understanding of its ultimate fate.

Other options describe phenomena that are not directly related to the role of dark energy. Black holes and dark matter, while significant in their own right, intertwine with different aspects of cosmic behavior and structure formation, but they do not explain the specific phenomenon of the universe's accelerated expansion. Additionally, the luminosity of stars is a product of nuclear fusion processes within those stars, not related to dark energy's influence on cosmic expansion. Understanding dark energy and its implications is crucial in modern astrophysics and cosmology.