Cosmic acceleration refers to the observation that the expansion of the universe is speeding up over time. This phenomenon was discovered in the late 1990s when astronomers studied distant supernovae, which are exploding stars. They found that these supernovae were dimmer than expected, indicating that the universe's expansion is not slowing down, as previously thought, but rather accelerating. The leading explanation for cosmic acceleration is the presence of dark energy, a mysterious force that makes up about 68% of the universe. Dark energy counteracts the force of gravity, causing galaxies to move away from each other at an increasing rate. Understanding cosmic acceleration is crucial for comprehending the ultimate fate of the universe.