Popcorn is far, far older than movies, and actually has a fairly interesting history.

I would highly recommend you watch the Filmmaker IQ video about the subject.

Just in case you can't watch the video, popcorn is among the oldest (if not the oldest) snack foods; evidence suggest it was consumed as far back as 4700 BCE. It became immensely popular in America in the late 1800s/early 1900s but wasn't initially accepted by the movie industry. Movie theaters were considered too high class for such a common snack food.

Popcorn's cheap cost allowed it to become a popular luxury during the Great Depression and its popularity was actually instrumental in keeping movie cinemas in business once they started selling it.