The Avengers will assemble once more in May, but their long separation has some fans wondering what Earth's mightiest heroes were up to in the meantime.

'Avengers 2' Tony Stark Is About To Make A Major Mistake, Find Out How Ultron Will Come Into Existence In 'Age Of Ultron'

Marvel filled Phase 2 with stand-alone films for Iron Man (Iron Man 3), Thor (Thor: The Dark World), and Captain America (Captain America: The Winter Soldier), but Caps (Chris Evans) was the only hero to receive significant aid from a fellow Avenger during his solo adventure.

While Black Widow (Scarlett Johansson) took up arms against HYDRA and the Winter Soldier (Sebastian Stan), four of her friends were conspicuously absent. Unfortunately for Avengers fans, Age of Ultron will not really explain why Hulk (Mark Ruffalo), Iron Man (Robert Downey Jr.), Captain America, Hawkeye (Jeremy Renner), and Thor (Chris Hemsworth) never collaborated following the Battle of New York.

Andy Serkis Teases 'Age Of Ultron' Role, Reveals Who He Is Not Playing

"Every time you do a stand-alone movie it's well, 'Why don't you call in the Hulk?'" Evans explained to Zap2It at the 2014 San Diego Comic-Con. "The fact is, we try to address this in the movie but if you want the stand-alone movies you've got to take it with a grain of salt."

If viewers are expecting an explanation that doesn't sound similar to, "Well, they all have their own lives and problems to attend to," than they will have to keep waiting.

"We try to give it that, at least one or two lines per movie. It's not easy, it's tricky. There are going to be some people who sit there with their arms crossed and expect something incredibly legitimate," Evans said. "Look, if you want these stand-alone movies, you've kind of just got to go along for the ride a little bit."

Find out more when The Avengers: Age of Ultron enters theaters on May 1.

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The Avengers 2, Avengers: Age of Ultron, Movies, Marvel