Dark flow refers to the observed coherent motion of galaxy clusters over vast scales, appearing as a bulk velocity of several hundred kilometers per second directed toward a specific region of the sky, extending out to distances of at least one billion light-years. This phenomenon was initially detected through measurements of the kinematic Sunyaev-Zel'dovich effect and has been controversial but persistent in various datasets, suggesting that large volumes of the observable universe are streaming in a common direction with velocities that cannot be explained by gravitational attraction from known structures within our cosmic horizon (Kashlinsky 2008; Watkins 2009). Lambda-CDM struggles with dark flow because the standard model predicts that on such large scales, matter should be approximately homogeneous and isotropic following Hubble expansion, with peculiar velocities caused only by local gravitational perturbations that should decay in amplitude with increasing scale; coherent flows extending across a billion light-years suggest either the presence of massive structures beyond our observable horizon pulling on our matter, or a fundamental breakdown of the cosmological principle that assumes large-scale homogeneity.