At the end of Avengers: Endgame, Steve Rogers travels back in time to return each of the Infinity Stones to the exact moment they were taken by the Avengers earlier in the film. Steve does this to prevent multiple alternate realities from existing — ones in which the stones were missing or absent from certain events.
It’s implied that Steve succeeds in doing this, but he never returns through the Avengers’ time travel machine. Instead, Steve returns as an old man and reveals that after he went back in time, he spent the next several decades enjoying his life. What Avengers: Endgame doesn’t explain, though, is whether Steve went back and reunited with Hayley Atwell’s Peggy Carter in the same timeline he’s always existed in, or went and found her in an alternate timeline.
Fortunately, one new fan theory may help clear up some of the confusion over Steve’s unseen time travel journey.
The Theory — A new from argues that Steve may have simply moved between two different timelines at the end of Avengers: Endgame — the one where he created a life for himself with Peggy and the prime reality where he had his meeting with Sam — without affecting his own past.
The Redditor believes that Steve’s time travel journey may have resembled the multiverse adventures that the characters on Agents of S.H.I.E.L.D. embarked on in the final season, which included plenty of timeline hopping. What’s more, the theory posits Steve may have even gotten in touch with some of the S.H.I.E.L.D. characters for help moving between the timelines.