The Milky Way ate another galaxy called Gaia-Enceladus, and the waves passing through a star have shown us that it happened at most 11.6 billion years ago
Source:
https://www.newscientist.com/article/2229836-a-single-star-has-let-us-put-a-date-on-our-galaxys-last-cosmic-meal/?utm_campaign=RSS%7CNSNS&utm_source=NSNS&utm_medium=RSS&utm_content=home