. ▁For ▁much ▁of ▁the ▁universe ' s ▁history ▁the ▁expansion ▁has ▁been ▁due ▁mainly ▁to ▁in ert ia . ▁The ▁matter ▁in ▁the ▁very ▁early ▁universe ▁was ▁flying ▁apart ▁for ▁unknown ▁reasons ▁( most ▁likely ▁as ▁a ▁result ▁of ▁cos mic ▁infl ation ) ▁and ▁has ▁simply ▁continued ▁to ▁do ▁so , ▁though ▁at ▁an ▁ever - de cre asing ▁rate ▁due ▁to ▁the ▁attract ive ▁effect ▁of ▁gravity . ▁ ▁In ▁addition ▁to ▁slow ing ▁the ▁overall ▁expansion , ▁gravity ▁causes ▁local ▁cl ump ing ▁of ▁matter ▁into ▁stars ▁and ▁galaxies . ▁Once ▁objects ▁are ▁formed ▁and ▁bound ▁by ▁gravity , ▁they ▁" drop ▁out " ▁of ▁the ▁expansion ▁and ▁do ▁not ▁subsequently ▁expand ▁under ▁the ▁influence ▁of ▁the ▁cosm ological ▁metric , ▁there ▁being ▁no ▁force ▁comp elling ▁them ▁to ▁do ▁so . ▁ ▁There ▁is ▁no ▁difference ▁between ▁the ▁in ert ial ▁expansion ▁of ▁the ▁universe ▁and ▁the ▁in ert ial ▁separation ▁of ▁nearby ▁objects ▁in ▁a ▁vac u um ; ▁the ▁former ▁is ▁simply ▁a ▁large - scale ▁ext rap olation ▁of ▁the ▁latter . ▁ ▁Once ▁objects ▁are ▁bound ▁by ▁gravity , ▁they ▁no ▁longer ▁re ced e ▁from ▁each ▁other . ▁Thus , ▁the ▁And rom eda ▁galax y , ▁which ▁is ▁bound ▁to ▁the ▁Mil ky ▁Way ▁galax y , ▁is ▁actually ▁falling ▁towards ▁us ▁and ▁is ▁not ▁expand ing ▁away . ▁Within ▁the ▁Local ▁Group , ▁the ▁gravit ational ▁interactions ▁have ▁changed ▁the ▁in ert ial ▁patterns ▁of ▁objects ▁such ▁that ▁there ▁is ▁no