Smart Hulk (Mark Ruffalo) could ultimately resolve the mystery surrounding Old Man Steve Rogers (Chris Evans) left by Avengers: Endgame. Marvel Studios wrapped up its first three Phases collectively known as the Infinity Saga through the 2019 blockbuster. But aside from resolving the debacle regarding Thanos (Josh Brolin) and the Infinity Stones, it also capped off some personal stories, including Captain America’s.

Unlike Iron Man (Robert Downey Jr.) and Black Widow (Scarlett Johansson) whose arcs finished with a heroic sacrifice, Steve survived the fight against the Mad Titan. However, he was tasked with one final mission — return all the collected Infinity Stones from the time heist so as not to create unwanted branches of timelines. He carried out his work successfully, albeit off-screen, but by the time he returned to the present MCU, he's significantly aged. It's implied that after doing the job, he went further back in time to the late '40s in order to reunite and, ultimately, live a full life with Peggy Carter (Hayley Atwell). It's a great ending to his arc and the Infinity Saga in general. But even as Steve has already retired and passed on the Captain America mantle to Sam Wilson (Anthony Mackie), the specifics of his final adventure remain a big mystery. 

Related: Avengers: Endgame Created One Branched Timeline That Steve Didn’t Fix

So while Marvel Studios is already in the midst of moving on by focusing on Phase 4, questions regarding the matter continue. Marvel Studios has been mum about it and every time their storytelling comes close to providing definitive answers, they veer away from the topic. But, if the MCU ever decides to finally address the lingering plotline, it can use Smart Hulk in doing so. After the defeat of Thanos, Smart Hulk operated the Quantum Realm tunnel to send Steve to his mission. While Sam and Bucky (Sebastian Stan) were there, only the original Avengers knew how the time-traveling really works since they experienced it first hand. If anything, Smart Hulk may have known more about it than the original Captain America, and about as much as the nanotech-wielding Tony Stark and Nebula (Karen Gillan) who grasped the idea from the get-go. So once Old Man Steve popped back to the main MCU timeline, the green giant would have a better understanding of what just happened. 

Smart Hulk, knowing first-hand how tricky Steve's final mission is, would've naturally been curious how it went and asked what brought about his significant aging. Since he was in direct contact with the Ancient One (Tilda Swinton) who first shared the dangers of simply plucking the Infinity Stones from their previous location, Bruce's alter-ego would've also asked questions about what she had to say when the Time Stone was returned. Overall, he would've been more prodding for information than Sam or Bucky since he was well aware of the possibilities, limits, and risks of time traveling. On the flip side, it's difficult to imagine Steve not answering, at least his fellow Avenger's technical queries. 

While Smart Hulk is not due for a solo project any time soon, his upcoming appearance in the She-Hulk Disney+ series could be an indication that he will still be in the MCU for quite some time. As one of the only three founding Avengers still left in the franchise, Marvel Studios may not be keen on retiring the character. So, if Marvel Studios finally decide to address this nagging mystery about Steve's final quest in Avengers: Endgame, chances are that he would be somehow involved.

More: Loki Theory: Why The TVA Had Every Infinity Stone Except The Mind Stone

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