This question already has an answer here:
Regarding this thread:
Why is the idea that the total energy in the universe "zero" so popular (re: Laurence Krauss) and why is the flatness of the universe used to back this up when, according to that post, an open universe would not conserve energy so the total energy of the universe cannot be "zero", can it? What's the energy being defined as zero and why is that energy used to predict things about the universe when "the" energy (as the term is used in that post) is not zero?
Additionally, I found one very good explanation here for this,
This seems to indicate that in flat space (the sort of space used in these zero energy universe theories) the mass can NOT be positive thus contradicting the notion that mass is positive and gravity negative and the whole thing winds up being zero. Did I interpret that correctly?
The MO question is cross-posted to Physics.SE here,