~10 billion years agoReputable sourceWell documented
The Milky Way's Great Merger
On the timeline · around ~10 billion years ago · The Age of Galaxies
What happened
Early in its life, the young Milky Way collided and merged with another large galaxy — an event called Gaia-Enceladus (the 'Sausage'). ESA's Gaia mission found its stars still moving on distinctive paths through our galaxy today.
Why it matters
This ancient merger reshaped the Milky Way, building much of its halo and helping form the disc that the Sun would one day join.
How we know
Gaia mapped tens of thousands of stars moving against the galactic grain — the fossil signature of the swallowed galaxy.