The third possibility actually means either non-local effects in non-local theories (Section 7.10), or the effect of additional fields in local modified gravity theories. The important difference with CDM is that these fields are not simply representative of collisionless massive particles, that their behavior is determined by the baryons in static configurations, and that they can be subdominant to the baryonic density. In theories where their energy density dominates that of baryons, these new fields then really act as dark matter in the early universe, which is also a possibility (see Section 7.6 and 7.9)