The chapter discusses the concept of dark energy, which is often referred to as a component of the universe that behaves similarly to Einstein's cosmological constant, Λ. Dark energy is hypothesized to be dynamical, evolving to its natural value of zero, and is believed to explain the observed acceleration of the universe's expansion. The authors review the observational evidence for dark energy, including the cosmic microwave background radiation, supernova observations, and the cosmic field defects. They also discuss the historical development of the idea, from Einstein's initial introduction of the cosmological constant to modern theories of inflation and dark energy. The chapter highlights the challenges in understanding the nature of dark energy and the potential for future observations to provide more insights into its properties.The chapter discusses the concept of dark energy, which is often referred to as a component of the universe that behaves similarly to Einstein's cosmological constant, Λ. Dark energy is hypothesized to be dynamical, evolving to its natural value of zero, and is believed to explain the observed acceleration of the universe's expansion. The authors review the observational evidence for dark energy, including the cosmic microwave background radiation, supernova observations, and the cosmic field defects. They also discuss the historical development of the idea, from Einstein's initial introduction of the cosmological constant to modern theories of inflation and dark energy. The chapter highlights the challenges in understanding the nature of dark energy and the potential for future observations to provide more insights into its properties.