  Recent astronomical observations of systems of dark matter, which have been
cited as providing possible support for self-interacting cold dark matter, may
provide evidence for the extra dimensions predicted by superstring scenarios.
We find that the properties of the required dark matter self-interaction are
precisely the consequences of a world with 3 large extra dimensions of size
\~1nm, where gravity follows the r^{-5} law at scales below ~1nm. From the
cross sections measured for various dark matter systems, we also constrain the
mass of dark matter particles to be m_x ~ 3*10^{-16} proton mass, consistent
with the mass of axions.
