Dark flow is an astrophysical term describing a possible non-random component of the peculiar velocity of galaxy clusters. The actual measured velocity is the sum of the velocity predicted by Hubble's Law plus a possible small and unexplained (or dark) velocity flowing in a common direction.
Does Dark flow exits ? If yes what is the cause of it?
Bothun,
This is not exactly the case, as the articles you mention refer to local gravitational inhomogeneities; i.e. up to a maximum distance of 80 Mpc with galaxies. The dark flow is inferred from the SZ-effect via the x-ray emitting gas in clusters and extends beyond 800 Mpc. It has been detected from WMAP to Planck and is not believed to be due to any local structures.
Furthermore, local flows are object dependent. For example, if one measures galaxies, they will arrive at different results when using clusters or SNIa (e.g. see http://phys.org/news/2011-09-evidence-spacetime-cosmological-principle.html).
The proposals for its existence in LCDM cosmologies range from an adjacent universe to other ad hoc ideas; none have been confirmed or thoroughly developed. There are also other observations to take into consideration such as CMB hemispherical power asymmetry, which demonstrate the universe may not be homogeneous at the largest of scales (which could also provide the illusion of accelerated metric expansion; see http://adsabs.harvard.edu/abs/2014AstRv...9c...4P).
1, I am completely aware of sample dependence on these results.
2. WMAP and Planck came well after the ground based work
3. I believe the original query was related to gravity maps and flows that result from those inhomogeneities on any scale.
Kashlinsky etal 2008, ApJ 686, L49 may be a better "modern source"
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