The trailer for Avengers: Infinity War has fans giddy with speculation about a potential surprise cameo in the upcoming sequel. With so much anticipation, fans are surely in for a treat when the movie is released this spring.

"#InfinityWar is going to be two and a half hours of characters introducing themselves to each other and I won't care one bit," wrote Twitter user Alex Zalben, no doubt making a reference to the seemingly endless amount of exposition and clunky character introductions that makes these movies so distinct. (Anyone who watched Justice League in theaters can attest to the silliness apparent in these types of superhero films.)

Zalben continued, giving us even more of a laugh.

Speaking of introductions... who could be making that surprise cameo?

The teaser trailer is... quite a tease.

Twitter is abuzz with fan theories, and Captain Marvel seems to be the focus of much of the speculation.

"Eagle-eyed viewers of the latest Avengers: Infinity War trailer spotted what seemed to be a brief glimpse of Brie Larson's Captain Marvel."

Captain Marvel... is she back? twitter.com

The evidence is pretty compelling.

"At 11 seconds in the #InfinityWar Super Bowl TV spot, is @brielarson's Captain Marvel standing right behind #CaptainAmerica? "

Brian Crespo isn't the only one who thinks Captain Marvel may make an appearance.

"I think I saw Captain Marvel," wrote Twitter user June Snuw.

jerm (@jermriley2) also seemed pretty convinced.

But other fans aren't so sure...

Said SuperGuttaZombi, "I'm calling it now that after Avengers Infinity War and Avengers 4 we'll have a completely new team of Avengers consisting of:
1. Captain Marvel
2. Bucky ( As Captain America )
3. Black Panther
4. Spider-Man
5. Antman
6. The Wasp
7. Doctor Strange"

"YEAH NO THAT LOOKS LIKE CAPTAIN MARVEL BUT I STILL HEAVILY DOUBT ITS HER," wrote another.

That's quite a lot of excitement for Captain Marvel, who will be played by Oscar winning actress Brie Larsen and appear in the eponymous Captain Marvel, a film all her own, come summer 2019. And to think that just last month, shortly after the Captain Marvel's cinematic look was revealed, people were questioning the choice of having her don a teal suit.

Here is the full official trailer for 'Avengers: Infinity War.' What do you think? Who could it be? Could fans be onto something? Or are they reaching really far? Sound off in the comments below.

'Avengers: Infinity War' hits theaters on May 4, 2018.