  The dark matter annihilation rate at small relative velocities can be
amplified by a large boost factor using various mechanisms, including
Sommerfeld enhancement, resonance enhancement, and Breit-Wigner enhancement.
These mechanisms all involve a resonance near the threshold for a pair of dark
matter particles. We point out that if the resonance is in the S-wave channel,
the mechanisms are equivalent sufficiently near the resonance and they are
constrained by universal two-body physics. The amplified annihilation rate
requires a corresponding amplification of the elastic scattering cross section.
If the resonance is a bound state below the threshold, it has an increased
lifetime that is inversely proportional to the square root of the binding
energy. Its spatial structure is that of two dark matter particles whose mean
separation is also inversely proportional to the square root of the binding
energy.
