Can the lumpy distribution of galaxies be reconciled with the smooth X-ray background?

The fluctuations in X-ray background intensity that would be generated by clusters or superclusters of galaxies exceed those observed. It may be that the X-ray background arises from low-luminosity objects which are distributed much more smoothly and are more numerous than bright optical galaxies. Hierarchical universe models in which the distribution of X-ray sources follows the cosmological mass distribution are ruled out.