  We argue that WIMP dark matter can annihilate via long-lived "WIMPonium"
bound states in reasonable particle physics models of dark matter (DM).
WIMPonium bound states can occur at or near threshold leading to substantial
enhancements in the DM annihilation rate, closely related to the Sommerfeld
effect. Large "boost factor" amplifications in the annihilation rate can thus
occur without large density enhancements, possibly preferring colder less dense
objects such as dwarf galaxies as locations for indirect DM searches. The
radiative capture to and transitions among the WIMPonium states generically
lead to a rich energy spectrum of annihilation products, with many distinct
lines possible in the case of 2-body decays to $\gamma\gamma$ or $\gamma Z$
final states. The existence of multiple radiative capture modes further
enhances the total annihilation rate, and the detection of the lines would give
direct over-determined information on the nature and self-interactions of the
DM particles.
