There’s a Thanos-sized hole in the Marvel Cinematic Universe, and we’re still not sure what supervillain will fill it. To be fair, Marvel has plenty of time to figure things out before Avengers 5, which probably won’t unite Earth’s mightiest heroes again for a few years at the very least.

But with Eternals on the horizon, is it possible this millennia-spanning adventure could be our first big hint at the MCU’s big bad? A new fan theory argues this exactly, while pointing to an overlooked moment from 2014’s Guardians of the Galaxy as our first big clue at who Marvel’s Thanos replacement might be.

On the Fan Theories subreddit, u/JayNotAtAl argues that 2021’s Eternals “will explain the death of the Celestials and perhaps hint at a future return.” What does this mean? Well, we know that the Celestials (god-like intergalactic beings invented by Marvel Comics legend Jack Kirby) will make an appearance in Eternals, which makes sense since they created both the Eternals and their enemies, the Deviants, by meddling with human evolution.

But because Eternals is expected to cover a wide swath of MCU history, it’s possible that, in the cinematic universe’s present, the Celestials could be long dead. After all, we know that one of them was already killed. In Guardians of the Galaxy, the gang visits Knowhere, a mining colony floating out in space that’s actually the severed head of a Celestial. Who or what killed it? Should the other Celestials be worried?

“I think that the Eternals will explain some major threat that occurred and resulted in the death of the Celestials and that threat will be the new big Thanos level threat of the MCU,” u/JayNotAtAl writes. “Alternatively, maybe the Celestials and the survivors are preparing for a comeback.”

As u/JayNotAtAl points out, Guardians was also one of the first movies to really explain what the Infinity Stones were for and what Thanos wanted to do with them. So it makes sense the same movie could be used to set up the next big supervillain.

What if, just like Thanos and those stones, some mysterious new villain has also been gathering power by killing the Celestials? If that’s the case, Eternals could provide some major clues about who in the Marvel Cinematic Universe is powerful enough to defeat a Celestial — and what they want with them.

Who will that villain be? We have no way of knowing just yet. In Marvel’s comics, plenty of characters have taken on the Celestials successfully, from Galactus to God Emperor Doom (a beefed-up multiverse version of Doom). It’s too soon to say just yet, but if the answer lies in Eternals, than we only have six months to wait before the Avengers 5 villain is revealed.

Eternals will be released on February 12, 2021.

Correction: An earlier version of this article mistakenly referred to Knowhere as Ego’s head, while in the MCU it is the head of an unknown Celestial. We regret the error. Thanks, Anthony!