The discovery that the expansion of the universe is accelerating, confirmed by independent studies of Type Ia supernovae in 1998, has provided the final piece in the cosmological puzzle. This acceleration is attributed to dark energy, a form of energy with negative pressure that causes the universe's expansion to speed up. The nature of dark energy remains a profound mystery, with potential implications for both astrophysics and particle physics. The article reviews the observational evidence for cosmic acceleration, the theoretical models proposed to explain it, and the key observational probes that will help resolve this enigma in the coming years. It discusses the role of dark energy in the universe's evolution, from the early hot and dense phase to the current era of accelerated expansion, and highlights the importance of understanding dark energy for the future of the universe. The review also covers the historical context, from Einstein's introduction of the cosmological constant to the current consensus on dark energy and the challenges it poses for our understanding of the cosmos.The discovery that the expansion of the universe is accelerating, confirmed by independent studies of Type Ia supernovae in 1998, has provided the final piece in the cosmological puzzle. This acceleration is attributed to dark energy, a form of energy with negative pressure that causes the universe's expansion to speed up. The nature of dark energy remains a profound mystery, with potential implications for both astrophysics and particle physics. The article reviews the observational evidence for cosmic acceleration, the theoretical models proposed to explain it, and the key observational probes that will help resolve this enigma in the coming years. It discusses the role of dark energy in the universe's evolution, from the early hot and dense phase to the current era of accelerated expansion, and highlights the importance of understanding dark energy for the future of the universe. The review also covers the historical context, from Einstein's introduction of the cosmological constant to the current consensus on dark energy and the challenges it poses for our understanding of the cosmos.