'The Walking Dead' spinoff may have just confirmed how the zombie apocalypse started and it's a nod to a joke creator Robert Kirkman previously made

• Warning: There are major spoilers ahead for "The Walking Dead: World Beyond" season one, episode seven, "Truth or Dare."
• On Sunday's episode, a marine tells his comrades he heard the zombie virus was started because of a space spore.
• Creator Robert Kirkman joked in January that the virus was started by the very same thing.
• Kirkman has previously said he knows exactly what started the zombie outbreak in "TWD," but he never planned to reveal the answer because "it's dumb."

It's taken 10 years, but we finally may know how the zombie outbreak started on "The Walking Dead."

During a flashback on Sunday's "TWD: World Beyond," a marine named Owens shares how he believes the zombie apocalypse happened to his comrades.

"I heard it came back in a rocket that it started in space," Owens said. "Somebody breathed it in, it turned their stomachs and then they got on a plane."

Owens is told to "shut up" and the "space spore" theory isn't mentioned again. It's not clear whether the marine's gossip was just a rumor or the actual cause of the zombie virus.

Earlier in the episode, a marine leader says the "source and nature of the current crisis could not be confirmed," so the space spore theory could be just that, a theory.

Regardless, fans may have noticed the significance of this small moment on Sunday's episode. It's a direct nod to something "TWD" comic creator Robert Kirkman said earlier this year.

The significance of the 'space spore' line: Creator Robert Kirkman said the same thing earlier this year

Back in January, a fan tweeted at Kirkman asking "what caused the zombies." The "TWD" creator gave a simple two word response: "Space spore."

Many weren't sure if Kirkman was joking or if he just decided to give away the answer now that the long-running comic series ended in July 2019. In April, when some media outlets started reporting Kirkman's "space spore" reveal as fact, Kirkman clarified that he wasn't being serious.

"It was a joke! I would never reveal something like this in a tweet!" Kirkman wrote.

So how did the outbreak start? 'The Walking Dead' universe is definitely taking the virus approach

It's unclear whether or not "TWDWB" was simply making a nod to Kirkman's joke, making a joke of its own, or was delivering an off-the-cuff answer to the show's longest lingering mystery.

Either way, "TWD" universe is definitely leaning into the virus explanation.

Earlier in the season, eagle-eyed viewers may have noticed a flyer young Elton picked up in a flashback on episode five referred to the crisis as an "outbreak." The evacuation flyer reads: "The public's vulnerability to this outbreak is undeniable. Many resources are invested in the problem, and a solution will be found."

Previously, we learned the Civic Republic Military (the group who took Rick years ago on "TWD") is testing the dead for, what appears to be, a cure to the zombie virus.

At the end of episode six, Dr. Belshaw looks at a study written by Dr. Leo Bennett on "advanced immunology" that shows he has been studying another global epidemic — the Black Death — to try and find a cure to the current situation.

Revealing the start of the zombie outbreak goes against everything Kirkman believed in

In Kirkman's comic series of the same name, he never reveals the cause of the zombie apocalypse.

At 2017's San Diego Comic Con, Kirkman told fans he knows what caused the virus in the comic series, but he was going to keep that to himself.

"I know what caused the virus in 'The Walking Dead,' but it's dumb. I'll never tell anyone," Kirkman told fans during a panel highlighting the comic creator.

Kirkman has said before that fans would never learn the cause of the outbreak because he didn't think it was important to the story.

"As far as digging down to actually find the smoking gun and realizing what that causes, it's really just unimportant to the overall story," Kirkman said in press notes reviewed by Insider ahead of the series premiere for "Fear the Walking Dead" in 2015. "If we were to do a companion to 'The Walking Dead' and it was about a bunch of scientists that were working to find the cure and finding out the origins, that would bore me to tears."

It's possible Kirkman saw the "space spore" line in "TWDWB" scripts and thought he would tweet it because he was pretty adamant about never revealing the cause of the outbreak.

Perhaps AMC is trying to get ahead of, what they see as, potential competition from HBO's upcoming "The Last of Us" ("TLOU") adaptation, which was announced in March. That series, based off of a popular 2013 video game, also follows a zombie outbreak and doesn't shy away from letting fans know the outbreak was caused by a mutated fungus that overtakes its host.

The game, which won multiple Game of the Year awards, takes players on a harrowing journey to get a young girl who may be the key to a cure to doctors across the country.

AMC's flagship "Walking Dead" series will end its run in late 2022 after 11 seasons. You can follow along with our "Walking Dead" coverage here.