Sagittarius existed in real dimensions but over the past approximately 6 billion years, Milky Way has been absorbing dwarf galaxy

Whether the Milky Way's early growth ‌was fueled ⁠by homegrown star formation or a galactic merger has been a matter of debate, according to researchers. The new study indicates that the merger infused the Milky Way early in its history with a large volume of stars and interstellar gas that feeds star formation, as well as the mysterious substance called dark matter.