  We consider a simple supersymmetric hidden sector: pure SU(N) gauge theory.
Dark matter is made up of hidden glueballinos with mass $m_X$ and hidden
glueballs with mass near the confinement scale $\Lambda$. For $m_X \sim
1\,\text{TeV}$ and $\Lambda \sim 100\,\text{MeV}$, the glueballinos freeze out
with the correct relic density and self-interact through glueball exchange to
resolve small-scale structure puzzles. An immediate consequence is that the
glueballino spectrum has a hyperfine splitting of order $\Lambda^2 / m_X \sim
10\,\text{keV}$. We show that the radiative decays of the excited state can
explain the observed 3.5 keV X-ray line signal from clusters of galaxies,
Andromeda, and the Milky Way.
