The Rees-Sciama effect and the primordial nucleosynthesis
  It is known that, theoretically, the Rees-Sciama effect may cause arbitrarily large additional redshifts in the cosmic microwave background radiation due to transparent expanding voids having sizes comparable with the size of horizon. Therefore, again theoretically, eventual huge voids existing immediately after the recombination may essentially change the predictions of the theory of big bang nucleosynthesis. If this eventuality holds, then the dark matter can be dominantly baryonic and, simultaneously, one can be in accordance with the predictions of primordial nucleosynthesis theory. Studying this eventuality one arrives at the result that the observed extreme isotropy of the cosmic microwave background radiation rejects the existence of any such huge voids, and hence this eventuality does not hold. 
Attila Meszaros (Charles University, Prague, Czech Republic)