What is the cause of "dark flow" in astronomy and astrophysics? (Suggested experiment)

v. 3 n. 15

"Dark flow: Is a non-spherically symmetric gravitational pull from outside the observable universe responsible for some of the observed motion of large objects such as galactic clusters in the universe?" *

"Astronomers have found that whole clusters of galaxies are moving in convoy towards a single point in the sky. This unexpected motion appears to be separate from the expansion of the universe and the researchers even suggest that a force beyond the visible universe is the culprit. The discovery has been named “dark flow”." **

While apparently a fine point in a subdiscipline of physics, the observation of "dark flow" touches on fundamentals of gravity and cosmology. The cosmological principle indicates that the Universe is similar on a sufficiently large scale, but dark flow would indicate otherwise, as suggested schematically in the cover image, where the dark oval is the non-conformist. With a conventionally assumed fundamentally attractive gravity, a mass outside the visible Universe but after the Big Bang is conventionally hypothesized to be pulling a group of clusters of galaxies that can be observed -- dark flow. Recall, a cluster of galaxies is a stable gravitational "family of galaxies" that move relative to one another, but member galaxies cannot leave the family. The dark flow in the problem statement is a series of these clusters (dark gray oval in the cover drawing) all being driven in the same peculiar direction inconsistent with the rest of the Universe, a motion apart from the general Hubble expansion. A possible answer, then, could be one of the following choices:

1. The series of clusters of galaxies taking part in the dark flow is being pulled from a mass outside the observable Universe, within the opaque part of the Universe (within 380,000 years of the Big Bang). This conventional view assumes a fundamentally attractive gravity among all masses regardless of scale -- Newton's original supposition exclusively from within Solar system observations without an explanation of the central principle of locality (direct physical contact).
2. This cluster series is being pushed by a repulsive gravitational effect from the observable part of the Universe. Proposed view where attractive effects among masses are only apparent at the cluster of galaxy scale and below, and fundamentally repulsive otherwise, where locality for Newtonian gravity was established. ***

The Universe on the large scale appears as a common polymer sponge of mostly spherical voids of various sizes, among which is the material aspect that is analogous to galactic superclusters. In forming the entity the driver is the voids, not the material. The arrangement is generally random, but if there is an unusually large void there would be an aspect of non-randomness. It is suggested that observation concentrate on identifying a possible unusually large (accelerated expanding) cosmic void on the trailing aspect of the dark flow.

* https://en.wikipedia.org/wiki/List_of_unsolved_problems_in_physics#Cosmology_and_general_relativity

** https://physicsworld.com/a/mysterious-dark-flow-at-the-edge-of-the-universe/

*** Cosmic inflation or gravity? | LinkedIn

Cover image caption: Schematic of dark flow