Scott Lang aka Ant-Man's (Paul Rudd) appearance in Avengers: Endgame first teaser trailer sparked several rumors about the smallest-Avengers. As per the latest Endgame movie theory, Ant-Man holds the key to solving all the Avengers, and he can also reverse the decimation which Thanos did in Avengers: Infinity War movie.
In Avengers: Infinity War movie, fans witnessed one of the most heartbreaking scenes when Thanos actually used all the Infinity Stones to wipe out half the Universe. In just a split second, viewers saw how superheroes like Black Panther, Spider-Man, Dr. Strange, and others were decimated.
One of the most burning questions after watching Avengers: Infinity War was why Scott Lang was not present during the Steve Rogers' fight with the army of Thanos in Wakanda. It was, however, revealed in the film that after the events of Captain America: Civil War, Scott Lang was house arrested.
Back in Ant-Man and the Wasp movie, it was shown how Scott Lang entered the quantum realm after he was asked by Dr. Hank Pym (Michael Douglas). Later in the scene, he couldn't communicate with Dr. Pym, Janet Van Dyne, and Hope because they all were decimated by Thanos' "snap."
However, Scott Lang did appear in the Avengers: Endgame trailer and it made many to believe that he can actually come out of the quantum realm as per his comfortability.
Meanwhile, it was revealed on multiple occasions that Dr. Strange's Time Stone may be the solution to reversing everything viewers witnessed in Infinity War. As earlier reported, there were speculations that the moment Ant-Man entered the quantum realm was the time when Thanos' snapped his Infinity Gauntlet. By doing this, Ant-Man got saved and now it looks like, he is the key to everything.
In the released teaser trailer of Avengers: Endgame, we saw that Scott Lang has escaped the quantum realm — as of now, we have no idea how he managed to do that — but after coming out from the realm, he instantly goes to the Avengers headquarters.
There were speculations as to why he chose to visit the Avengers headquarters in the first place. Maybe, after coming out of the quantum realm, he realized what has actually happened on the planet Earth and he knew that if someone knows the answer then it would be the Avengers.
Furthermore, after he reaches the Avengers facility, we see Captain America asking Black Widow asking, "Is this an old message?"
As per Independent, in the video featuring Lang at the facility, there is a date stamp that shows 1983. So, it means that instead of ending up the present era, somehow Ant-Man went to 1983. If this speculation is true then Ant-Man has traveled time and might be all alone. At the same time, if he knows about Thanos' action from the future, he would actually let all the other Avengers know and save the entire Universe.
Avengers: Endgame movie is slated for release on April 26, 2019.