How can galaxies collide if everything moving outwards If the universe started from Big Bang and everything is expanding outwards and actually accelerating away from each other, than "How is it possible for two galaxies to collide as they all are moving in the same direction". To collide they must in opposite direction!
Edit:How is it possible for two galaxies to collide as those colliding galaxies are moving in the same direction.
 A: Galaxies interact with each other due to their mutual gravitation.
If matter had been exactly evenly distributed after the big bang then galaxies wouldn't collide, but there wouldn't be any galaxies anyway. The inhomogeneities in the matter distribution are believed to originate from quantum fluctuations that occurred when inflation ended. Some regions ended up with higher than average matter density and some with lower than average matter density. In the overdense regions the mutual gravity of the matter overcame the expansion of spacetime and those regions collapsed to form galaxy superclusters, then clusters then galaxies. Within a galaxy cluster the galaxies have essentially random velocities relative to each other, which is why there are sometimes collisions.
A: The expansion of the universe is such that the space added in-between A and B is such that A and B get further apart at rate, say R, proportional to the distance between A and B. To be precise R = H*D, where D is distance and H is a constant.  This is Hubble's law. Therefore the further apart A and B are, the faster they will appear to move away from each other.  
Now the above is assuming A and B are stationary relative to one another.  It is possible for B to collide with A, if A is moving at a velocity greater R in the direction of B.
