The development of ferromagnetic domains in exchange-coupled ferromagnetic/antiferromagnetic $\text{Fe}(2.6\text{ }\text{nm})/{\text{FeF}}_{2}(50\text{ }\text{nm})$ bilayers is studied by magneto-optical magnetometry and microscopy in a temperature range from 300 to 17 K. At room temperature, the cubic anisotropy of the single-crystal Fe can be clearly identified from the formation of characteristic domain patterns. With the onset of exchange bias, below the N\'eel temperature of ${\text{FeF}}_{2}$, the density of the nuclei of patchlike magnetic domains continuously increases with increase in exchange bias and the congruent increase in coercivity. The increase in nucleation density is interpreted as a local increase in the density of pinned interfacial moments. As changes in the bulk antiferromagnetic structure are excluded from field history dependent measurements, the temperature dependent exchange bias is shown to be directly connected to a lateral inhomogeneous increase in the interfacial exchange coupling.