However , on smaller length-scales , matter is observed to form ` clumps ' , many atoms are condensed into stars , most stars into galaxies , most galaxies into galaxy groups and clusters and , lastly , the largest-scale structures such as the Great Wall of galaxies . However , on smaller length-scales , matter is observed to form `` clumps '' , i.e. , to cluster hierarchically ; many atoms are condensed into stars , most stars into galaxies , most galaxies into clusters , superclusters and , finally , the largest-scale structures such as the Great Wall of galaxies .