The Marvel universe is so packed with detail that some concepts are just as important to understand as people or places. One such scientifc concept present in the MCU are the iconic Pym Particles, which are key to the narratives of Ant-Man and Avengers: Endgame. Pym Particles first appeared on-screen in 2015's Ant-Man, which depicted their initial discovery by scientist Hank Pym. That film provided a great showcase for the particle's capabilities, and their drawbacks as they became the primary item of interest in multiple films.
The primary usage of Pym Particles is to change the physical size and subsequently alter the strength and resilience of the user. Pym also invented the high-tech suits which allowed them to harness the particles, he and his wife used them as weapons in their actions for S.H.I.E.L.D. Tragically, his wife Janet van Dyne was forced to overuse the particles to avert disaster, resulting in her sudden and prolonged disappearance. This event traumatized Pym, leading him to attempt to permanently hide away the particles.
Pym's protege, Darren Cross, later forced Pym out of the company and began recreating the particles for sale to ill-intentioned buyers. Pym was forced to hire a new wielder of his old suit to stop this from happening, finding his hero in expert thief Scott Lang. Lang successfully defeated Cross and brought the Pym Particles back into use for good, leading to Hank Pym's daughter Hope taking her mother's mantle.
After a couple of fun adventures with the pair, The Mad Titan Thanos achieved his dark goal completely unknown to Ant-Man and The Wasp. Thanos' snap wiped out half of all life in the universe, and unfortunately, Hank, Janet and Hope were all killed, leaving Lang alone. Luckily, brilliant scientists Tony Stark and Bruce Banner were able to use Lang's limited supply of Pym Particles for a previously unthinkable purpose; time travel. In order to undo Thanos' actions, The Avengers create new suits to harness the particles for their new purpose.
Technically, the particles allow the heroes to travel to an alternate timeline, which they intend to repair after taking from. Their first target is, cleverly, alternate Hank Pym's lab in 1970 to gather more Pym Particles for more trips. The alternate universe's Thanos, by capturing Nebula, acquires a mote of the treasured particles for himself, whereupon his minions engineered them to put Thanos in place for the film's grand final battle.
So how are these subatomic particles able to change the mass of matter, adjust density with absurd limits, trap its users in limbo and travel through time and alternate universes? Because Pym Particles hail from the Quantum Realm, so their applications all occur through access to that alternate dimension. Mass is reduced by shunting the excess to the alternate dimension, that's why reducing size too far results in semi-permanent entrapment within the Quantum Realm.
The particles can be used for time travel by using the Quantum Realm as a bridge. After Tony Stark develops a method of finding pathways through the dimension, The Avengers are able to briefly enter the Quantum Realm and exit it in a different universe, time and space. Essentially, Pym Particles are the first human method of approaching an alternate dimension that has always existed beyond the veil, and their discovery offered new ways of interacting with our reality through a new one.
Pym Particles are similar, but not identical, to their original source material. The first Marvel Comics appearance of the Pym Particles came in Tales to Astonish issue 27, way back in 1962. This series was an anthology horror comic, drastically different in tone from Marvel's typical output today. In the story entitled "The Man in the Ant Hill", scientist Henry Pym discovers Pym particles which can be used to shrink matter.
After testing it on himself, Pym finds himself being chased and hunted by ants, now made terrifying in scale. Pym manages to escape and immediately destroys the substance. Years later, a narrative which would inspire the MCU's take on the concept would become canon. Since the late 90's, the Pym Particle of the comics has also worked through dimensional travel, specifically to a dimension called Kosmos. This take features a more lively reality than the Quantum Realm's glowy etherealness.
The upcoming third film in the Ant-Man franchise is subtitled Quantumania. This certainly implies that the Quantum realm will play a central role in the narrative of this upcoming blockbuster. A scene in the previous feature, Ant-Man and the Wasp, saw the team rescue Janet van Dyne from this alternate dimension. Despite this delve into the Quantum Realm, its involvement in the narrative is still limited. Its more of a narrative tool than a setting. With the upcoming film's subtitle and the world Ant-Man still has open to explore, its safe to assume Pym Particles will remain a key concept in the MCU for many films to come.