  We show that viable electroweak baryogenesis can be realized without a
first-order phase transition if plasma is heated inhomogeneously by
nongravitational structure formation in some particle species. Yukawa
interactions can mediate relatively long-range attractive forces in the early
Universe. This creates an instability and leads to growth of structure in some
species even during the radiation dominated era. At temperatures below the
electroweak scale, the collapsing and annihilating halos can heat up plasma in
fireballs that expand and create the out-of-equilibrium high-temperature
environment suitable for generating the baryon asymmetry. The plasma
temperature at the time of baryogenesis can be as low as a few MeV, making it
consistent with both standard and low-reheat cosmologies.
