  We consider a phenomenological continuum theory for an extensile, overdamped
active nematic liquid crystal, applicable in the dense regime. Constructed from
general principles, the theory is universal, with parameters independent of any
particular microscopic realization. We show that it exhibits a bend instability
similar to that seen in active suspensions, that leads to the proliferation of
defects. We find three distinct nonequilibrium steady states: a defect-ordered
nematic in which $+\frac{1}{2}$ disclinations develop polar ordering, an
undulating nematic state with no defects, and a turbulent defective nematic. We
characterize the phenomenology of these phases and identify the relationship of
this theoretical description to experimental realizations and other theoretical
models of active nematics.
