Movement through space is inevitable. Right now, you’re sitting on a planet spinning on its axis, moving you at up to 1,600 km/h depending on your latitude. The Earth orbits the Sun, the Sun moves through the Milky Way, the Milky Way moves through the Local Group of galaxies, and the Local Group moves through our local Supercluster. Even that Supercluster of galaxies is in motion, heading towards the Shapley Supercluster, where scientists believe a particularly large concentration of galaxies exists. I hope all this motion isn’t making you too dizzy.
On the grandest scales of our universe, something strange is happening. Scientists have realized there is not enough mass in the Shapley Supercluster to pull us towards it at the rate we observe—about 50% of the cause of that motion is unaccounted for. In 2017, researchers discovered a possible source for the rest of that motion, but it’s not something pulling us; it’s something pushing. Join me today as we explore the evidence for the region of space known as the Dipole Repeller and try to understand how, in a universe filled with gravity, something can push instead of pull.
It has taken a long time for astronomers to recognize this motion. To understand that we were moving, scientists first had to map the regions of space around us. While we’ve been mapping stars since the beginnings of civilization, it wasn’t until Edwin Hubble's discovery in 1924 that we confirmed the existence of stars in other galaxies beyond the Milky Way. Before then, astronomers believed the fuzzy clusters of light in the night sky were spiral nebulae within the Milky Way rather than independent galaxies. Hubble’s observation of Cepheid stars, a type of variable star with a predictable luminosity cycle, allowed him to calculate their distance accurately, proving they lay beyond the Milky Way.
Since then, astronomers have worked to map galaxies and superclusters, identifying the vast structures of the universe. By the 1970s, Stephen Gregory, Laird Thompson, and William Tifft had shown that galaxies converged into Superclusters—large filamentary or sheet-like structures surrounded by vast cosmic voids. They mapped the first Supercluster, the Coma Supercluster, spanning a region of space 100 million light-years across. A staggering 95% of all galaxies reside within such superstructures. However, it wasn’t until 2014 that our own supercluster, the Laniakea Supercluster, was fully mapped.
You might expect mapping regions of space closer to us to be easier than mapping distant galaxies, but the opposite is true due to the Milky Way itself. The Milky Way is filled with gas and dust, creating a Zone of Avoidance where light struggles to penetrate. This cosmic fog makes it difficult to observe objects behind it, including key regions of our supercluster. Distant galaxies are easier to study because telescopes can focus on single points in the sky without obstruction. Mapping nearby galaxies requires observing the entire night sky, a much more complex task.
Thanks to the study of galaxies’ peculiar motion—motion relative to the cosmic background radiation that ignores universal expansion—scientists have grouped galaxies into Superclusters. At the center of our Laniakea Supercluster lies the Great Attractor, an area 150-250 million light-years away that the Milky Way is drifting towards. However, the Great Attractor lies within the Zone of Avoidance, making it difficult to study clearly.
Further complicating matters, the Laniakea Supercluster itself is moving towards another supercluster, the Shapley Supercluster, which also lies in the Zone of Avoidance. Despite improvements in x-ray and infrared telescopes, the mass within these superclusters still doesn’t fully explain our observed motion. In 2017, a team of researchers from the University of Hawaii, including Yehuda Hoffman, Daniel Pomarède, R. Brent Tully, and Hélène M. Courtois, published a groundbreaking study in Nature to explain this discrepancy. They created a map emphasizing galaxies' motions rather than just their positions, which revealed a surprising discovery: mass was not only being drawn toward the Shapley Supercluster but also moving away from another region.
This mysterious region was named the Dipole Repeller. Together, the Dipole Repeller and the Shapley Attractor account for approximately 50% of our galaxy’s motion. But how can something push in a universe where gravity only pulls? Could it be a collection of white holes or some source of anti-gravity? The answer, surprisingly, is nothing.
The Dipole Repeller is a void, a cosmic bubble about 100 million light-years across, nearly devoid of galaxies. It exists between the filaments of the universe’s structure. Our galaxy’s motion aligns more closely with being pushed away from this void than being pulled toward the Shapley Attractor, supporting the theory of the Dipole Repeller. But how can nothing exert a push?
The explanation involves a pseudo-force. Imagine a universe where galaxies are evenly spaced. Gravity from galaxies above, below, and on all sides would balance out, leaving you stationary. Now, if galaxies were removed from one direction, the balance would shift, and you would move away from the void as though being pushed. This gravitational imbalance creates the illusion of a repelling force.
Another contributing factor may be cosmic expansion. The universe’s expansion, which appears to be accelerating, is counteracted by gravity. In regions with more matter, expansion slows due to gravitational binding. However, in a void where gravity is minimal, expansion continues unchecked, causing void regions to swell compared to their denser supercluster counterparts. This differential expansion may create a repelling effect, further contributing to our galaxy's motion.
Since 2017, further research on the Dipole Repeller has been limited due to the Zone of Avoidance and the inherent difficulty of studying an absence rather than a presence. However, Tully and his team remain hopeful that future ultra-sensitive surveys across multiple light spectrums will help identify the few galaxies within this void and confirm its influence.
We are constantly moving through space, and so is the science that studies it. Each new discovery, from galaxy clusters to cosmic voids, helps us better understand the forces shaping our universe. Fascinatingly, not all cosmic motion seems to be caused by gravity's pull—sometimes, the universe itself gives a little push.