Here's why:
Direction and Speed: Andromeda is currently moving towards us, and its trajectory indicates a direct collision.
Gravitational Pull: The galaxies' mutual gravitational attraction is pulling them closer together.
Historical Evidence: Galactic collisions are a common occurrence in the universe.
While the collision may seem like a catastrophic event, it's important to remember that galaxies are mostly empty space. The chances of individual stars colliding are very low. However, the collision will likely result in the merging of the two galaxies, creating a new, larger galaxy.