  It has been suggested recently that self-interacting dark matter fits better
the observational characteristics of galaxy dynamics. We propose that the
self-interacting dark matter is composed from the glueballs of the hidden
sector non-Abelian gauge group, while the hidden matter states exist in
vector-like representation and decouple from the light spectrum. It is shown
that these glueballs are semi-stable with the life-time larger than the present
age of the Universe, if their mass is 1 GeV or less. The constraint on their
abundance today suggests that the energy was stored in the hidden sector soon
after inflation. This imposes an upper limit on the reheating temperature. We
further study the naturalness of this scenario in the context of the
free-fermionic string models and point out a class of such models where the
self-interacting dark matter from the hidden sector is indeed plausible.
