Where did the dense mass come from that started the Big Bang?
Physicists seem to think that it wasn't a single dense mass. Instead, so far as they can figure out from the mathematics, energy in the curvature of space was converted into particles just the way we can stimulate the physical vacuum today to disgorge particle-antiparticle pairs 'out of thin air'.
Curved space (the gravitational field of the cosmos) supplied the energy to do this trick at the Big Bang. You didn't need a single massive object to do it. Matter appeared, and literally leaked, out of the vacuum...everywhere there was space and time.
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