Our Milky Way is colliding with the Canis Major dwarf galaxy as we speak.

http://www.universetoday.com/guide-to-spac...-way-collision/

But it won't be as catastrophic as the big one, a collision between our galaxy and Andromeda Galaxy, ETA about 3 billion years from now.

http://www.haydenplanetarium.org/resources...s/G0601andmilwy

Anything can happen before that: collision with comets, scientists creating a singularity by accident, etc. All devastating to human life.