Galaxies are far more than the sum of their stars. Long before stars even formed, dark matter clumped up and drew regular matter together with its gravity, providing the invisible scaffolding upon which stars and galaxies eventually grew.

Today, nearly all galaxies are still embedded in giant “halos” of dark matter that extend far beyond their visible borders and hold them together, anchoring stars that move so quickly they would otherwise break out of their galaxy’s gravitational grip and spend their lives adrift in intergalactic space.

The way dark matter and stars interact influences how galaxies change over time. But until recently, scientists had mainly only examined one side of that relationship, exploring the way dark matter pulls on normal matter.

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It's been textbook knowledge for over a century that our Milky Way galaxy is doomed to collide with another large spiral galaxy, Andromeda, in the next 5 billion years and merge into one even bigger galaxy. But a fresh analysis published in the journal Nature Astronomy is casting that longstanding narrative in a more uncertain light. The authors conclude that the likelihood of this collision and merger is closer to the odds of a coin flip, with a roughly 50 percent probability that the two galaxies will avoid such an event during the next 10 billion years.

Both the Milky Way and the Andromeda galaxies (M31) are part of what's known as the Local Group (LG), which also hosts other smaller galaxies (some not yet discovered) as well as dark matter (per the prevailing standard cosmological model). Both already have remnants of past mergers and interactions with other galaxies, according to the authors.

"Predicting future mergers requires knowledge about the present coordinates, velocities, and masses of the systems partaking in the interaction," the authors wrote. That involves not just the gravitational force between them but also dynamical friction. It's the latter that dominates when galaxies are headed toward a merger, since it causes galactic orbits to decay.

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