Someone asked the same question. Found it.
Wolfgang524 ? 6 months ago ?
So if the universe is expanding with every galactic object moving away from each other, then how do galaxies collide?

Andrew Adkins ? 6 months ago ?
The universe is expanding, but not in a straight line - There are closely packed galactic clusters that swirl around each other at the same time the group is moving away from the center of the universe. Take our solar system for example - the planets orbit the Sun as it orbits Sagittarius A (the black hole at the center of the Milky way). It's sort of the same thing, but on a much larger scale.

http://www.wired.com/wiredscience/20...paign=Previous