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Published online by Cambridge University Press: 04 August 2017
Once it is agreed that not all mass gives a significant contribution to light or any other emission, then one must rely on the dynamics of the visible objects to determine the total gravity field. It is clearly impossible to do this without subsidiary hypotheses. Here we shall assume that all members of the Local Group began together in the Big Bang, and that their dynamics have been governed by their mutual gravitational interactions since the system first achieved a size of some 200 kpc. This must have been some 109 years after the Big Bang.