Final answer:
The Andromeda Galaxy (M31) is expected to collide with the Milky Way in about 4 billion years, leading to their merger into a larger galaxy.
Step-by-step explanation:
The galaxy expected to collide with the Milky Way someday is the Andromeda Galaxy (M31). According to astronomical research and computer models, this monumental event is projected to take place in about 4 billion years. The interaction between the Milky Way and the Andromeda Galaxy will occur over a complex series of events, ultimately resulting in their merger to form a larger, more rounded galaxy.