  We present kinematically forbidden dark matter annihilations into Standard
Model leptons. This mechanism precisely selects the dark matter mass that gives
the observed relic abundance. This is qualitatively different from existing
models of thermal dark matter, where fixing the relic density typically leaves
open orders of magnitude of viable dark matter masses. Forbidden annihilations
require the dark matter to be close in mass to the particles that dominate its
annihilation rate. We show examples where the dark matter mass is close to the
muon mass, the tau mass, or the average of the tau and muon masses. We find
that most of the relevant parameter space can be covered by the next generation
of proposed beam-dump experiments and future high-luminosity electron positron
colliders. Forbidden dark matter predicts large couplings to the Standard Model
that can explain the observed value of $(g-2)_\mu$.
