The morphologies and sizes of micellar aggregates, composed of the tri-block copolymer P123 (EO20PO70EO20) in a mixture of the aprotic solvent N,N-dimethylformamide (DMF) and water, were investigated by combining Dynamic Light Scattering (DLS) and Cryogenic Transmission Electron Microscopy (cryo-TEM) experiments. At water concentrations between about 27 and 35 wt% bicontinuous micelles with distinct patterns were formed, in coexistence with very long, non-branched, worm-like micelles. Water concentration affects both the size and the morphology of the micellar aggregates. A careful study of the pure binary solvent mixture revealed the presence of dynamic solvent domains of nanometric size, even in the absence of copolymer. Strikingly, the size of these solvent nano-domains closely matched the size of the bicontinuous micelles in a polymer solution for the same water/DMF ratios. We discuss these findings in terms of spinodal decomposition of the polymer solution, in which two-solvent domains contain solvent quality fluctuations that could determine the decomposition. In addition, we suggest another "soft confinement" mechanism that could be responsible for the formation of bicontinuous micelles. The local excess of one of the solvent species in the nano-domains could entrap a metastable morphology.