The cosmic event know as the "The Snap!" or later the Decimation was caused by the mad Titan; Thanos. He hoped to to appease his love the entity known as Death, Thanos used the Infinity Gauntlet with all of the Infinity Gems to remove half of the population of the Universe.
Marvel Cinematic Universe (Earth-199999)
Thanos obtained Infinity Gauntlet and use the Infinity Gems. He then proceeded to Snap! his fingers to destroy half the universe. He later destroyed the gauntlet so nobody could reverse what he had done. 
Five years later, Scott Lang returned from the Quantum Realm. He met with the Avengers and formed a plan to go back in time to rebuild the Gauntlet, first using it to restore those killed by Thanos' usage, then to wipe out the past Thanos from and his horde.
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