2012A&A...539A...4C

The cosmological mean matter (dark and baryonic) density measured in the units of the critical density is Ω_m=0.27. Independently, the local mean density is estimated to be Ω_loc=0.08-0.23 from recent data on galaxy groups at redshifts up to z=0.01-0.03 (as published by Crook et al., 2007ApJ...655..790C and Makarov & Karachentsev, 2011MNRAS.412.2498M). If the lower values of Ω_loc are reliable, as Makarov & Karachentsev and some other observers prefer, does this mean that the Local Universe of 100-300Mpc across is an underdensity in the cosmic matter distribution? Or could it nevertheless be representative of the mean cosmic density or even be an overdensity due to the Local Supercluster therein. We focus on dark matter halos of groups of galaxies and check how much dark mass the invisible outer layers of the halos are able to host. The outer layers are usually devoid of bright galaxies and cannot be seen at large distances. The key factor which bounds the size of an isolated halo is the local antigravity produced by the omnipresent background of dark energy. A gravitationally bound halo does not extend beyond the zero-gravity surface where the gravity of matter and the antigravity of dark energy balance, thus defining a natural upper size of a system. We use our theory of local dynamical effects of dark energy to estimate the maximal sizes and masses of the extended dark halos. Using data from three recent catalogs of galaxy groups, we show that the calculated mass bounds conform with the assumption that a significant amount of dark matter is located in the invisible outer parts of the extended halos, sufficient to fill the gap between the observed and expected local matter density. Nearby groups of galaxies and the Virgo cluster have dark halos which seem to extend up to their zero-gravity surfaces. If the extended halo is a common feature of gravitationally bound systems on scales of galaxy groups and clusters, the Local Universe could be typical or even an overdense region, with a low density contrast ∼1.