▁of ▁the ▁star ▁and ▁ σ ▁is ▁the ▁Stefan – B olt z mann ▁constant , ▁with ▁a ▁value ▁of ▁▁ ▁Thus , ▁when ▁both ▁lum inos ity ▁and ▁temperature ▁are ▁known ▁( via ▁direct ▁measurement ▁and ▁calculation ) ▁the ▁radius ▁of ▁a ▁star ▁can ▁be ▁determined . ▁ ▁Gal ax ies ▁The ▁spect ra ▁of ▁galaxies ▁look ▁similar ▁to ▁stell ar ▁spect ra , ▁as ▁they ▁consist ▁of ▁the ▁combined ▁light ▁of ▁bill ions ▁of ▁stars . ▁ ▁Do pp ler ▁shift ▁studies ▁of ▁galax y ▁clusters ▁by ▁Fritz ▁Z wick y ▁in ▁ 1 9 3 7 ▁found ▁that ▁the ▁galaxies ▁in ▁a ▁cluster ▁were ▁moving ▁much ▁faster ▁than ▁seemed ▁to ▁be ▁possible ▁from ▁the ▁mass ▁of ▁the ▁cluster ▁infer red ▁from ▁the ▁visible ▁light . ▁Z wick y ▁hypoth es ized ▁that ▁there ▁must ▁be ▁a ▁great ▁deal ▁of ▁non - l umin ous ▁matter ▁in ▁the ▁galax y ▁clusters , ▁which ▁became ▁known ▁as ▁dark ▁matter . ▁Since ▁his ▁discovery , ▁astronom ers ▁have ▁determined ▁that ▁a ▁large ▁portion ▁of ▁galaxies ▁( and ▁most ▁of ▁the ▁universe ) ▁is ▁made ▁up ▁of ▁dark ▁matter . ▁In ▁ 2 0 0 3 , ▁however , ▁four ▁galaxies ▁( N GC ▁ 8 2 1 , ▁NGC ▁ 3 3 7 9 , ▁NGC ▁ 4 4 9 4 , ▁and ▁NGC ▁ 4 6 9 7 ) ▁were ▁found ▁to ▁have ▁little ▁to ▁no ▁dark ▁matter ▁influen cing ▁the ▁motion ▁of ▁the ▁stars ▁contained ▁within ▁them ; ▁the ▁reason ▁behind