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.