We formulate the theory of incoherent dark spatial solitons in noninst
antaneous self-defocusing nonlinear media. We find that the basic moda
l constituents of these incoherent dark soliton entities are radiation
modes as well as bound states. Our results explain for the first time
why incoherent dark solitons are in fact gray and why a transverse pi
-phase flip can facilitate their observation.