The annihilation cross section of the dark matter is one of the most
important quantity for the phenomenology of the dark matter.
If the dark matter couples with a light force carrier,
non-perturbative resummation is required in non-relativistic regime
due to so-called Sommerfeld enhancement.
A commonly used formula of this cross section is a product of the hard
(short-range) scattering cross section and the enhancement factor
which is calculated by dark matter two-body wave function.
However, it is known that this formula violates the unitarity bound
for small velocity if there is zero energy bound state.
In this talk, I will discuss the calculation of the dark matter
annihilation cross section in non-relativistic quantum mechanics.
In our framework, both short-range and long-range effect is treated in
a non-perturbative way,
and the cross section satisfies the unitarity bound.