This factual statement is a metaphor, which implies being alone, in danger, and in a seemingly hopeless situation.
It
originally entered pop culture as the tag line on the Movie Poster for James Cameron's Alien (1979).
This statement is an oversimplification of fact. The vacuum of outer space transmits sound poorly; however, to scream, you need air, and thus others on
your spaceship would hear you scream were you able.
This
expression emphasizes the terrifying truth that we are truly alone when travelling through outer space. No help is nearby. You and your crew have to be self-reliant.
For its time, Alien was exceptionally scary and realistic, thus the expression is burnt in the American psychic with a sizable connotation of fear, hence the words are memorable to this day.
