Why can't we hear sound in space?

Short answer: Because sound needs a medium (particles) to travel. If there are no particles to carry pressure waves, no sound reaches a listener — and if there’s no listener, that’s a separate reason, but not the fundamental one.

Explanation:

• Sound is a mechanical wave: it’s a pattern of compressions and rarefactions traveling through matter (gas, liquid, or solid). Those require particles (molecules, atoms) bumping into each other.
• Deep interplanetary and interstellar space is extremely low density (a vacuum for practical purposes). There aren’t enough particles to transmit ordinary audible pressure waves, so sounds produced by, say, an explosion don’t propagate as sound over any significant distance.
• If a sound source existed in rock or inside a spacecraft, the sound would travel through that material and could be heard by someone in contact with it. Conversely, if there’s truly no medium between source and listener, no sound travels, so even if a listener were present, they wouldn’t hear it.
• So: the primary reason is lack of a medium to carry the sound. The absence of a listener only matters insofar as you’re asking whether anyone hears anything — but physics doesn’t depend on an observer; without particles, there’s no sound wave to detect.