That surprising twist at the end of the Avengers: Endgame trailer had fans crossing their fingers that the superheroes are not dead. That they are after all alive, but just in an alternate reality.
Scott Lang's appearance at the end of the Avengers: Endgame trailer had fans coming up with different theories why he survived the Thanos snap and how he managed to escape the Quantum Realm in Avengers: Infinity War. The end credits scene clearly showed the Ant-Man trapped inside with no other means of escape since his friends have disintegrated. So how he ended up outside the Avengers headquarters then is still a mystery.
However, a Marvel fan and Reddit user proposed an interesting theory that could explain Scott Lang's appearance. The fan, who goes by the name of "joethehamface," said that no one died in the Thanos snap, otherwise known as The Decimation. Instead, they are in a different reality. The Mad Titan did not kill them but simply split the reality into two.
Those who turned to dust are in a different reality than the surviving heroes and vice versa. Each reality have missing people, and the heroes saw each other disintegrate. If Captain America, Iron Man, and Black Widow saw their friends die, then Scarlet Witch and the rest also saw those dubbed "survivors" vanish before their eyes. In these two realities, both parties are trying their best to find ways to bring everyone back alive.
This possibility was even hinted in a scene in Avengers: Infinity War when Thanos talked about death. He may have been speaking literally when he said that death is futile if anyone has the capability to change things with the powers of the Infinity Gauntlet. He had the stones at his disposal, and so he merely changed reality and not kill 50 percent of the human population.
Now, Scott Lang figures into this theory because he was in the Quantum Realm during The Decimation. The realm is its reality or dimension, so the snap did not affect him. He might even exist in both realities.
This is where Scott Lang plays a major role in Avengers: Endgame. The trailer shows he brought his van along with him, which he has equipped with a portal to the Quantum Realm. The way to bring back all the heroes together is supposedly to bridge the two realities through the realm. However, whether this theory is right remains to be seen when the film hits theaters on April 26, 2019.
