If there is a single underlying "theory of everything" which in some limits
of its "moduli space" reduces to the five weakly coupled string theories i
n 10D, and 11D SUGRA, then it is possible that all six of them have some co
mmon domain of validity, and that they are in the same universality class,
in the sense that the 4D low-energy physics of the different theories is th
e same. We call this notion string universality. This suggests that the tru
e vacuum of string theory is in a region of moduli space equally far (in so
me sense) from all perturbative theories. most likely around the self-dual
point with respect to duality symmetries connecting them. We estimate strin
gy nonperturbative effects from wrapped brane instantons in each perturbati
ve theory, show how they are related by dualities, and argue that they are
likely to lead to moduli stabilization only around the self-dual point. We
argue that moduli stabilization should occur near the string scale, and SUS
Y breaking should occur at a much lower intermediate scale, and that it ori
ginates from different sources. We discuss the problems of moduli stabiliza
tion and SUSY breaking in currently popular scenarios, explain why these pr
oblems are generic, and discuss how our scenario can evade them. We show th
at string universality is not inconsistent with phenomenology, but that it
is in conflict with some popular versions of brane world scenarios.