  Cannibals are dark matter particles with a scattering process that allows
three particles to annihilate to two. This exothermic process keeps the gas of
the remaining particles warm long after they become non-relativistic. A
cannibalizing dark sector which is decoupled from the Standard Model naturally
arises from a pure-glue confining hidden sector. It has an effective field
theory description with a single massive interacting real scalar field, the
lightest glueball. Since warm dark matter strongly suppresses growth of
structure cannibals cannot be all of the dark matter. Thus we propose a
scenario where most dark matter is non-interacting and cold but about 1 percent
is cannibalistic. We review the cannibals' unusual scaling of the temperature
and energy and number densities with redshift and generalize the equations for
the growth of matter density perturbations to the case of cannibals. We solve
the equations numerically to predict the scaling of the Hubble parameter and
the characteristic shape of the linear matter power spectrum as a function of
model parameters. Our results may have implications for the $\sigma_8$ and
$H_0$ problems.
