To the Moon:

The phrase “To the Moon and Back” is a widely recognized cultural expression most famous for its association with immense love and affection, though it also serves as the title for several prominent books, songs, and media projects. The Famous Idiom & Origin

The expression “I love you to the moon and back” is an idiomatic metaphor used to describe a love so vast that it spans the roughly 400,000-mile round-trip distance between Earth and the moon.

Pop Culture Origin: The phrase gained its massive global popularity from Sam McBratney’s classic 1994 children’s book, Guess How Much I Love You. In the story, Little Nutbrown Hare tells his father he loves him “right up to the moon,” and Big Nutbrown Hare replies, “I love you right up to the moon — and back.”

Heart Energy Theory: A popular romanticized piece of trivia suggests the phrase mirrors the fact that the human heart generates enough daily energy to drive a truck 20 miles. Over an average lifespan, that total energy equates to a driving distance equivalent to traveling to the moon and back. Literature

Several notable authors have published books under this specific title: What it means to say “to the moon and back” – Steemit