The most notorious Grease fan theory suggests that Olivia Newton-John’s Sandy is in fact dying throughout the movie and the infamous teenage love story is no more than a fantasy. Starting out as a raunchier stage play than the eventual adaptation, Randal Kleiser’s 1950s set high-school musical was a mostly grounded navigation through the struggles of typical teenage years. While it has its moments of fantasy, the ending, in which Danny and Sandy fly off to their hopeful future always felt like it didn’t belong.

https://screenrant.com/grease-movie-sandy-dead-theory-explained/