Void Galaxy Distribution: A Challenge for $\Lambda$CDM
  We extract void catalogs from the Sloan Digital Sky Survey Data Release 16
(SDSS DR16) survey and also from the Millennium simulation. We focus our
comparison on distribution of galaxies brighter than $M_r < -18$ inside voids
and study the mean separation of void galaxies, distance from the void center,
and the radial density profile. We find that mean separation of void galaxies
depends on void size, as bigger voids have lower mean separation in both
samples. However, void galaxies in the observation sample seem to have
generally larger mean-distance than simulated ones at any given void size. In
addition, observed void galaxies tend to reside closer to the void center than
those in the simulation. This discrepancy is also shown in the density profile
of voids. Regardless of the void size, the central densities of real void
profiles are higher than the ones in the predicted simulated catalog.
