▁scale ▁of ▁galax y ▁clusters ▁and ▁above ), ▁because ▁gravit ational ▁att raction ▁bind s ▁matter ▁together ▁strongly ▁enough ▁that ▁metric ▁expansion ▁cannot ▁be ▁observed ▁at ▁this ▁time , ▁on ▁a ▁smaller ▁scale . ▁As ▁such , ▁the ▁only ▁galaxies ▁re ced ing ▁from ▁one ▁another ▁as ▁a ▁result ▁of ▁metric ▁expansion ▁are ▁those ▁separated ▁by ▁cosm olog ically ▁relevant ▁scales ▁larger ▁than ▁the ▁length ▁scales ▁associated ▁with ▁the ▁gravit ational ▁collapse ▁that ▁are ▁possible ▁in ▁the ▁age ▁of ▁the ▁universe ▁given ▁the ▁matter ▁density ▁and ▁average ▁expansion ▁rate . ▁ ▁Phys ic ists ▁have ▁post ulated ▁the ▁existence ▁of ▁dark ▁energy , ▁appearing ▁as ▁a ▁cosm ological ▁constant ▁in ▁the ▁simplest ▁gravit ational ▁models , ▁as ▁a ▁way ▁to ▁explain ▁the ▁acceleration . ▁According ▁to ▁the ▁simplest ▁ext rap olation ▁of ▁the ▁currently - f av ored ▁cosm ological ▁model , ▁the ▁L ambda - CD M ▁model , ▁this ▁acceleration ▁becomes ▁more ▁dominant ▁into ▁the ▁future . ▁In ▁June ▁ 2 0 1 6 , ▁NASA ▁and ▁E SA ▁scient ists ▁reported ▁that ▁the ▁universe ▁was ▁found ▁to ▁be ▁expand ing ▁ 5 % ▁to ▁ 9 % ▁faster ▁than ▁thought ▁earlier , ▁based ▁on ▁studies ▁using ▁the ▁Hub ble ▁Space ▁T eles cope . ▁ ▁While ▁special ▁relativ ity ▁prohib its ▁objects ▁from ▁moving ▁faster ▁than ▁light ▁with ▁respect ▁to ▁a ▁local ▁reference ▁frame ▁where ▁spac etime ▁can ▁be ▁treated ▁as ▁flat ▁and ▁un ch anging , ▁it ▁does ▁not ▁apply ▁to ▁situations ▁where ▁spac etime ▁curv ature ▁or ▁evolution