 A particular reason why the wave function of the Universe cannot be the whole ontology lies in the argument, led by Maudlin 2010, that this is the wrong type of object. The wave function of the Universe (considered to have N particles) is defined in 3N dimensional configuration space, while we need an entity in 3+1 space-time (like the primitive ontology), see discussion by Albert 1996, Lewis 2004, Monton 2006, Ney 2021. Addition of “primitive ontology” to the wave function of the Universe helps us understand our experience, but complicates the mathematical part of the theory. In the framework of the MWI, it is not necessary. The expectation values of the density of each particle in space-time, which is the concept derived from the wave functions corresponding to different worlds, can play the role of “primitive ontology”. Since interactions between particles are local in space, this is what is needed for finding causal connections ending at our experience. The density of particles is gauge independent and also properly transforms between different Lorentz observers such that they all agree upon their experiences. In particular, the explanation of our experience is unaffected by the “narratability failure” problem of Albert 2013: the wave function description might be different for different Lorentz frames, but the description in terms of densities of particles is the same. Note also an alternative approach based on 3+1 space-time by Wallace and Timpson 2010 who, being dissatisfied with the wave function ontology, introduced the formulation of  Spacetime State Realism. Recently more works appeared on this subject: Ney and Albert 2013, Myrvold 2015, Gao 2017, Lombardi et al. 2019, Maudlin 2019, Chen 2019, Carroll and Singh 2019. These works show significant difficulties in obtaining our world as emergent from the Universal wave function. This explains the skeptical tone of  Everett’s relative-state formulation of quantum mechanics.  But, as discussed in Sec.3, the success of the “emergence” program is not crucial: it is enough to find the counterpart of the world we experience in the Universal wave function.