  Fock and Goncharov conjectured that the indecomposable universally positive
(i.e., atomic) elements of a cluster algebra should form a basis for the
algebra. This was shown to be false by Lee-Li-Zelevinsky. However, we find that
the theta bases of Gross-Hacking-Keel-Kontsevich do satisfy this conjecture for
a slightly modified definition of universal positivity in which one replaces
the positive atlas consisting of the clusters by an enlargement we call the
scattering atlas. In particular, this uniquely characterizes the theta
functions.
