Chloe Bennet, Clark Gregg

Ever since Agents of S.H.I.E.L.D. was picked up, there's been one question on everyone's minds: How the heck is Agent Coulson still alive after being brutally stabbed through the heart in The Avengers?

Slowly, but surely, the ABC super series has unraveled that mystery. Thus far, we've learned that Coulson (Clark Gregg) was dead for several days, not a few minutes, and that Director Fury (Samuel L. Jackson) moved heaven and Earth to bring him back to life through several surgeries that also wiped his memories of the experience. More recently, Coulson discovered a secret bunker that housed half a blue alien whose blood, imbued with regenerative powers, saved Coulson's life. It was the same blood that was used to bring Skye (Chloe Bennet) back to life.

Agents of S.H.I.E.L.D.: Which teammember has been hiding a big secret?

So what the heck was that alien? Theories have run wild online, with Lady Sif (Jaimie Alexander) even rattling off a list of various species that could be the culprit in the most recent episode. "I try to figure it out," Bennet said during Sunday's PaleyFest panel, explaining that she continuously scours the Internet for answers. She guesses that the Kree might have something to do with it.

However, the writers played coy when TVGuide.com pressed for more answers on the red carpet. "There are a lot of theories, and with Guardians of the Galaxy coming out, people are steering one way because everything is connected in the Marvel Universe," executive producer Maurissa Tancharoen tells us. The film will feature both the Kree race, with Lee Pace playing Ronan the Accuser in the film, and the Centaurians, with Michael Rooker playing the blue-skinned Yondu of the Zatoan tribe. "We've heard a lot of theories and some of them are correct," executive producer Jed Whedon added.

Fortunately, we bribed tricked convinced executive producer Jeff Bell to debunk one theory. "It's not a Frost Giant," he revealed. "We had to talk with the Marvel Studio because they've got aliens coming up in some of their movies, so we had to make sure that we fit within that, and we wanted to do it in a limited way. We're not saying there are these people running around on the planet. There's this thing that could've been there for thousands of years that they found."

Agents of S.H.I.E.L.D.: What really saved Coulson and Skye?

Suffice it to say, Simmons (Elizabeth Henstridge) and Fitz (Iain de Caestecker) will go to great lengths to figure out what that serum was that saved Skye's life, even if it means breaking some rules — which might uncover even more dangerous secrets. "She's got an inner battle going on," Henstridge tells us. "The oath that she took as a scientist is that she wants to help people, so she's going to go as far as she can, but she doesn't want to endanger the team or herself by digging into stuff that she's not equipped to handle. She loves Skye and cares about Skye. She's doing it for Skye, but also doing it as part of her job. She wants to figure out if this can help other people."

Regardless of what the blue alien is, the implications of Skye being an 0-8-4 who now has an unknown alien substance in her has led fans to believe that she could eventually become a superhero — maybe even Ms. Marvel, who gained her powers as a result of being in an explosion with Kree superhero Captain Marvel. After all, we don't know Skye's real name. For all we know, it could be Carol Danvers.

What do you think the blue alien is? Hit the comments!

Marvel's Agents of S.H.I.E.L.D. airs Tuesdays at 8/7c on ABC.