Dark energy is the dominant component of the universe, accounting for about 76% of its total energy content, and is responsible for the observed accelerated expansion of the universe. This discovery, made in 1998 through observations of distant Type Ia supernovae, confirmed that the universe is not only expanding but doing so at an increasing rate. This finding has led to a deeper understanding of the universe's composition, which includes 4% baryonic matter, 20% dark matter, and 76% dark energy. The nature of dark energy remains one of the greatest mysteries in modern cosmology, with implications for both astrophysics and particle physics. The discovery of cosmic acceleration has been supported by multiple lines of evidence, including observations of the cosmic microwave background (CMB), large-scale structure, and galaxy clusters. These observations have confirmed that the universe is spatially flat and that dark energy is the dominant component. The CMB provides a record of the universe at a simpler time, before structure had developed, and its anisotropies offer insights into the universe's composition and evolution. Large-scale structure observations, such as baryon acoustic oscillations (BAO), also provide evidence for dark energy, as they reveal the distribution of matter in the universe. Theoretical models of dark energy include the cosmological constant, which is the simplest explanation of accelerated expansion, and modified gravity theories that suggest General Relativity may break down on cosmological scales. The cosmological constant, originally introduced by Einstein to allow a static universe, has been re-evaluated in light of the observed accelerated expansion. However, the energy density of the quantum vacuum is estimated to be much larger than observed, leading to the cosmological constant problem. The study of cosmic acceleration has led to a better understanding of the universe's expansion history and the role of dark energy in shaping it. Observational probes such as supernovae, weak gravitational lensing, and galaxy clusters are being used to further constrain the properties of dark energy. Future surveys, including ground-based and space-based missions, will provide even more precise measurements, helping to determine the nature of dark energy and its role in the universe's fate. The ultimate goal is to understand whether dark energy is a form of vacuum energy or something else, and whether General Relativity accurately describes the universe's expansion.Dark energy is the dominant component of the universe, accounting for about 76% of its total energy content, and is responsible for the observed accelerated expansion of the universe. This discovery, made in 1998 through observations of distant Type Ia supernovae, confirmed that the universe is not only expanding but doing so at an increasing rate. This finding has led to a deeper understanding of the universe's composition, which includes 4% baryonic matter, 20% dark matter, and 76% dark energy. The nature of dark energy remains one of the greatest mysteries in modern cosmology, with implications for both astrophysics and particle physics. The discovery of cosmic acceleration has been supported by multiple lines of evidence, including observations of the cosmic microwave background (CMB), large-scale structure, and galaxy clusters. These observations have confirmed that the universe is spatially flat and that dark energy is the dominant component. The CMB provides a record of the universe at a simpler time, before structure had developed, and its anisotropies offer insights into the universe's composition and evolution. Large-scale structure observations, such as baryon acoustic oscillations (BAO), also provide evidence for dark energy, as they reveal the distribution of matter in the universe. Theoretical models of dark energy include the cosmological constant, which is the simplest explanation of accelerated expansion, and modified gravity theories that suggest General Relativity may break down on cosmological scales. The cosmological constant, originally introduced by Einstein to allow a static universe, has been re-evaluated in light of the observed accelerated expansion. However, the energy density of the quantum vacuum is estimated to be much larger than observed, leading to the cosmological constant problem. The study of cosmic acceleration has led to a better understanding of the universe's expansion history and the role of dark energy in shaping it. Observational probes such as supernovae, weak gravitational lensing, and galaxy clusters are being used to further constrain the properties of dark energy. Future surveys, including ground-based and space-based missions, will provide even more precise measurements, helping to determine the nature of dark energy and its role in the universe's fate. The ultimate goal is to understand whether dark energy is a form of vacuum energy or something else, and whether General Relativity accurately describes the universe's expansion.