Panspermia (from Ancient Greek πᾶν (pan) 'all', and σπέρμα (sperma) 'seed') is the hypothesis that life exists throughout the Universe , distributed by space dust , meteoroids , asteroids , comets , planetoids , and also by spacecraft carrying unintended contamination by microorganisms . Distribution may have occurred spanning galaxies , and so may not be restricted to the limited scale of solar systems .