In our extragalactic neighborhood lies a vast empty region that might exert a repulsive force on our galaxy, the Milky Way. This “repeller” contributes to the gravitational forces that drive us at a speed of nearly 2.3 million km/h through the cosmic web, the large-scale structure on which matter is organized and comprising filaments that interconnect galaxies and separate voids.

The movement of our galaxy (and its companion, the Andromeda galaxy) was already familiar to astrophysicists, who had been searching for its cause for 40 years. Researchers first sought an explanation in the possible existence of an excess of galaxies located in the general direction of our motion. The initial suspect was called the Great Attractor, a region containing half a dozen galaxy-rich clusters and located 150 million light years away. Attention then shifted to a larger entity, still in the same line of sight but directly behind the Great Attractor, known as the Shapley Supercluster, located 600 million light years away from us. Over the years, however, the debate became stuck on the relative importance of these two attractors, neither of which offers a sufficient explanation for our movement, particularly as we are not moving exactly in the direction of Shapley, as we should be doing. The hypothesis of a sub-dense region or extragalactic “void” was then put forward to shed light on this phenomenon.

It is, however, extremely difficult to confirm the observation of such a void. Which is why the researchers decided that instead of focusing on just the visible part of the mass - in other words the galaxies - they should map out in 3D the movement of all matter, namely visible or baryonic matter and invisible matter, known as dark matter, although it is in fact transparent. 

CEA¹ research engineer, Daniel Pomarède, explains: “It is microwave radiation emitted more than 13 billion years ago that enables us to detect the movement of the Milky Way. This light reaches us from every direction, but because we are moving, it is observed with a blue shift in the direction we are heading, and with a redshift in the opposite direction. And we can analyze this dipole effect to deduce our speed of 630 km/s.” Hélène Courteous, an astrophysicist at Université Lyon 1², adds: “By analyzing the velocity fields of thousands of galaxies in our local universe, we identified the movement of “rivers of matter”, such as those flowing through Lamiae,” our super cluster of galaxies discovered by the same team in 2014. “These rivers are a direct consequence of the distribution of total mass, which moves away from empty regions and towards those with higher densities.” 

The team discovered the presence in our galaxy of repulsive and attractive forces of comparable strength coming from distant entities. It deduced that the major influences behind our movement are the Shapley Attractor, and a vast, previously unidentified, empty region (i.e., devoid of visible and invisible matter) that they have called the Dipole Repeller. This discovery, published on January 30, 2017 in Nature Astronomy, could indeed explain the existence of the dipole (see box below) observed in the cosmic wave background, one of the vital components of the standard model of cosmology.

Astrophysicists have finally learned what path our galaxy is following, as it is pulled by the distant Shapley Supercluster, and repelled by the Dipole Repeller, a region made up of “nothing”, not even invisible matter, and about which almost nothing is known. That is why astrophysicists are now preparing ultrasensitive optical, near-infrared and radio readings to identify the rare galaxies that may be found in and around such a void to increase our knowledge.