The Avengers’ victory over Thanos in Avengers: Endgame was owed to their use of the Quantum Realm to travel through time and reacquire the Infinity Stones - but the MCU almost omitted it, raising the question of how this plot would have worked without a crucial element. The Quantum Realm was introduced to the MCU in 2015’s Ant-Man as a hazardous state of reality and potential consequence of using Hank Pym’s size-altering Pym Particles without failsafe measures in place.
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