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Although we can't feel it, we're in a constant state of motion.

The Earth spins at 1,600 km/h while orbiting the Sun at 100,000 km/h; the Sun orbits the Milky Way at 850,000 km/h; and the Milky Way moves in time with the expanding Universe at a staggering 630 km per second. But what is propelling the Milky Way's race through space?


Scientists had assumed a dense region of the Universe called the Great Attractor, made of clusters of galaxies 150 million light-years from the Milky Way, was pulling the galaxy through space. Astronomers later suggested an area of more than two dozen rich clusters, called the Shapley Concentration 600 million light-years beyond the Great Attractor, was to blame.

Now, researchers led by professor Yehuda Hoffman at the Hebrew University of Jerusalem report that our galaxy is not just being pulled, it's also being pushed. In a study in the journal Nature Astronomy, the team describes a previously unknown region in our extragalactic neighbourhood, largely devoid of galaxies, which exerts a repelling force on the Milky Way as well as Andromeda.

"By 3D mapping the flow of galaxies through space, we found that our Milky Way galaxy is speeding away from a large, previously unidentified region of low density. Because it repels rather than attracts, we call this region the Dipole Repeller," explained professor Hoffman. "In addition to being pulled towards the known Shapley Concentration, we are also being pushed away from the newly discovered Dipole Repeller. Thus it has become apparent that push and pull are of comparable importance at our location."


The presence of such a low-density region had been suggested previously, but confirming the absence of galaxies has, until now, proved difficult. Using the Hubble Space Telescope, among other powerful telescopes, the team designed a 3D map of the so-called galaxy flow field.

Flows directly respond to the distribution and movement of matter away from regions that are relatively empty, toward regions that are full of mass. The researchers used the map to study the peculiar velocities - those faster than the Universe's rate of expansion - of galaxies around the Milky Way to discover regions that are densely populated "attract" this flow, and the less dense ones repel it.

By identifying the Dipole Repeller, the researchers could determine both the direction and size of the Milky Way's motion. Future research will use ultra-sensitive surveys at optical, near-infrared and radio wavelengths to identify the small number of galaxies in this void, and confirm the Dipole Repeller.