Most of the universe is slipping away from the Milky Way, carried outward as space itself expands. Yet one giant neighbor is bucking the trend, racing straight at us while almost every other major galaxy recedes. That lone holdout is Andromeda, and astronomers are finally piecing together why gravity has overruled cosmic expansion in this one spectacular case.
The answer ties together the discovery that galaxies are flying apart, the hidden influence of dark energy, and the local tangle of gravity inside our small corner of the cosmos. It also sets the stage for the Milky Way’s distant future, when our sky will be dominated not by retreating galaxies, but by a single sprawling merger remnant.
The universe is expanding, but our neighborhood is complicated
To understand why almost every major galaxy is fleeing the Milky Way, I have to start with the basic rule of the cosmos: on large scales, space is stretching. When astronomer Edwin Hubble measured the light from distant galaxies, he found that they are, on average, moving away from us, a result that supported the hypothesis that the universe is expanding and cemented the modern picture of the Big Bang driven by Edwin Hubble. The farther a galaxy is, the faster it recedes, which is why deep surveys show a cosmos where nearly all bright galaxies are redshifted and drifting away.
On top of that expansion sits dark energy, a mysterious component that is uniform in space but very weak, yet still strong enough on the largest scales to account for the observed acceleration of the expansion. As one physics explainer notes, dark energy loses out to ordinary forces in small regions, where gravity and even chemical bonds easily overpower it. That is why atoms, planets and galaxies do not get torn apart by expansion, even as galaxy clusters and superclusters drift apart across tens of billions of light years.
The Local Group: a small island in a receding sea
The Milky Way does not sit alone in this expanding universe, it lives in a relatively small group of about 50 g of galaxies known as the Local Group. Our galaxy, the Milky Way, is one of the two dominant spirals in this association, the other one being the Andromeda Galaxy, which together anchor a swarm of smaller companions inside a shared gravitational well described in detail by Our Local Group. Within this island of matter, gravity is strong enough to bind galaxies together, so the global expansion of space does not simply pull them apart.
Beyond the Local Group, however, the story changes. On scales larger than this modest cluster of roughly fifty systems, the pull of the Milky Way and Andromeda fades, and the Hubble expansion takes over, so more distant galaxies recede from us with increasing speed. In that sense, almost every major galaxy you might name, from the Virgo Cluster giants to remote quasars, is effectively fleeing the Milky Way, not because they are firing rockets away from us, but because the fabric of space between our Local Group and the rest of the universe is steadily stretching.
Andromeda: the one big galaxy heading straight for us
Inside this gravitational bubble, one object stands out. The Andromeda Galaxy, also known as M31, is not receding, it is moving toward us at about 110 km/s, a motion that has been clocked in multiple observations and highlighted in public explainers that note how the Andromeda galaxy will collide with our Milky Way because of this approach speed and the enormous mass of the Andromeda system. At a distance of roughly 2.5 million light years, that speed is enough to overcome the gentle push of cosmic expansion, so gravity wins and the two spirals are on a converging path.
Seen from Earth, this neighbor is already impressive. It has a comparable width to the Milky Way but M31 has roughly twice the star count, and as some observers note, the Milky Way and Andromeda together will eventually become a giant lenticular or elliptical galaxy once they merge, a fate often illustrated using long exposure images of Andromeda. For now, though, it remains a faint smudge to the naked eye, even as it rushes closer at a staggering speed that some outreach pieces translate to about 250,000 miles per hour when describing how the Milky Way and Andromeda are moving toward each other in a cosmic dance captured in Milky Way and.
Gravity versus expansion: why Andromeda is the exception
The key to Andromeda’s odd behavior lies in the tug of war between local gravity and the global expansion of space. At the modest separation of a few million light years, the combined mass of the Milky Way and Andromeda is strong enough that gravity beats the universe’s expansion, so instead of drifting apart, the pair is drawn together as a bound system, a point that is often summarized by noting that The Milky Way and Andromeda are massive enough that their mutual pull overwhelms expansion at this distance in explanations of why Milky Way and are converging. Dark energy still permeates this region, but its effect is simply too feeble on such scales to counteract the attraction of hundreds of billions of stars and vast halos of dark matter.
That is why outreach materials emphasize that dark energy is uniform in space but very weak, and that gravity and other forces like chemical bonds easily overpower its influence in smaller systems, even as it accounts for the observed acceleration of the expansion on the largest scales, a contrast spelled out in discussions of Gravity and dark energy. In other words, Andromeda is not defying the laws of cosmology, it is simply close enough, and massive enough, that the usual rule of receding galaxies gives way to the more familiar rule of falling objects.
What the coming merger will look like from Earth
Over the next several billion years, that inward motion will reshape the night sky. Our neighbor, the Andromeda Galaxy, is rushing toward us under the stronger force of our mutual gravitational attraction, and outreach guides describe how, over the ages, it will grow ever larger in the night sky as it approaches, turning today’s faint smudge into a dominating feature as Andromeda Galaxy swells. Some visualizations invite readers to look up at the night sky and then imagine doing the same 4 to 5 billion years from now, when that smudgy galaxy will have stretched across a huge swath of the heavens as it moves closer to us at about 110 km/s, a scenario vividly described in videos that urge viewers to Look Now.
Simulations suggest that in about 4 to 5 billion years, the two galaxies will begin to merge, with tidal forces pulling the structures of both galaxies apart and reshaping their spiral arms into long streams of stars, a process that some explainers summarize by noting that From Earth, Andromeda will grow larger in the night sky as the merger unfolds over billions of years, a vision captured in descriptions of how From Earth the view will change. Educational videos walk through what happens when two galaxies meet, explaining that in about 4.5 billion years they will collide and eventually settle into a surprisingly graceful merger of two giant galaxies, a fate charted in resources that invite viewers to consider What happens when Andromeda and the Milky Way collide and in courses that chart the fate of the Milky Way as Andromeda speeds toward it for a surprisingly graceful merger of two giant galaxies using Hubble imagery.
More from Morning Overview
*This article was researched with the help of AI, with human editors creating the final content.
