5.2. Diffuse Light
Zwicky 's original approach to the "missing mass" problem consisted in looking for the missing light. He was the first to claim detection of large luminous patches in the centre of Coma. Later on, Welch & Sastry , Kormendy & Bahcall , Thuan & Kormendy  and Mattila  also found evidence for diffuse light in the centre of Coma. The contribution of this diffuse component to the total cluster mass was however estimated to be negligible (at most 3 % of the total mass, according to Kormendy & Bahcall).
Mattila proposed several possible origins for this diffuse light: extended galaxy envelopes, dwarf galaxies (see Section 4), globular clusters, intergalactic stars, scattering by dust grains (see Section 5.4).
Recently, based on deep CCD observations, Bernstein et al. showed that the diffuse light in Coma follows the same distribution of globular clusters and dwarf galaxies, and the units that make up such a diffuse luminosity must be 103 L.