The Milky Way is destined to collide and merge with the Andromeda galaxy.

Andromeda is moving toward the Milky Way, but whether the galaxies actually collide depends on Andromeda’s much smaller sideways motion. In 2012, Hubble measurements constrained that motion well enough that NASA said astronomers could predict the future collision “with certainty.” Simulations placed the first major encounter about four billion years ahead and a later merger several billion years after that.

The prediction became a familiar picture of the distant future, but it depended on measurements and galaxy masses that still carried substantial uncertainty.

A 2025 analysis combined newer Hubble and Gaia data with updated mass estimates and the gravitational effects of M33 and the Large Magellanic Cloud. Instead of evolving only one best-estimate Local Group, the researchers sampled the measurement uncertainties across many possible starting conditions.

The result was much less deterministic. In the full four-galaxy model, only about half of the simulations produced a Milky Way–Andromeda merger within the next 10 billion years. M33 tends to increase the chance of a merger, while the Large Magellanic Cloud can pull the Milky Way onto a less collision-prone path.

Andromeda is still approaching, and a merger remains entirely possible. What changed is the certainty: the collision is now one plausible future rather than an event astronomers can treat as inevitable.