Is every aspect (fundamental forces, periodic table) of an antimatter universe exactly the same, except the fact that it's all antimatter?

Question

Are all the constants (Boltzmann's, gravitational, $c$) the same? Would elements react differently? Would it look the same?

Answer 1

Almost, but not quite. The answer to the questions you ask

Are all the constants (Boltzmann's, gravitational, c) the same? Would elements react differently? Would it look the same?

are all "yes", but various processes controlled by the weak nuclear interaction violate charge symmetry (sometimes known a C-symmetry) and would be different.

Most of those can be restored however by both exchanging matter for antimatter (C) and reflecting the universe in a mirror (P, for "parity").

However, there are a few processes which violate that combined symmetry as well. Notably among them is the decay of the neutral kaon. (A fact discovered in 1964, BTW.) More recently some B-meson decays have also been explicitly measured to violate time-reversal symmetry (T), which should happen if it is true the the combination of all three operators (CPT) represents a true symmetry.

Answer 2

(C)harge conjugation + (P)arity transformation + (T)ime reversal - CPT - is observed to be an exact symmetry of nature (as far as anyone can tell so far anyway). Applying the C, P and T operators to the entire Universe would exchange every particle for its antiparticle (and also mirror-image everything, and flip all momenta). Provided CPT is in fact an exact symmetry of the Universe, the CPT-flipped Universe would be indistinguishable in terms of its physics from the "normal" Universe.

By the way, "time reversal" does NOT mean run time backwards - it has a precise definition in terms of how certain physical quantities (e.g. momentum) transform when T is applied.