Why doesn't sound travel in space?

Sound travels through a medium (i.e., air, water, metal pipe, etc.). The particles of the medium vibrate in waves

of sound, and it is the vibrations of the molecules that we perceive as sound. Space is a vacuum (nearly), and

there is no medium in which sound can travel (the space between galaxies contains about 1 hydrogen atom per

cubic meter, and the space inside of galaxies contains about 1 hydrogen atom per cubic centimeter).