Why do galaxies collide?
The sound waves in the presence of the moving object in its back will be compressed and will reach our ears at very high frequency. If these sound waves are present behind the moving object then they are expanded and will sound with deeper tone. This is explained clearly in Doppler Effect. The theory of big bang is supported by this concept of Doppler Effect. It also reveals that Universe is expanding. The changes in expansion of Universe are observed by red shift or calculus. The Hubble expansion of the Universe indicates that Universe is expounding and the Galaxies are also departing from each other.
If the Universe is unfolding and the galaxies are moving apart further, then is there any possibility for galaxies to collide with one another? There is an opinion that the galaxies will collide when they are attracted by gravitational pull. But this opinion cannot be considered valid in one perspective as the gravitational force that exists between all the matter might have made the Universe to get united after the Big Bang before the galaxies are formed. But, it can also be thought that the trajectory formation might have made the objects in the Universe to remain away from one another in a given time.
If the gravitational pull concept is correct, then the galaxies must be moving with certain acceleration in order to not collide with each other. When the Space is expanding, the local gravity can keep those galaxies to exist together and overcome the unfolding of the Universe. Some of the Galaxies like Andromeda and few others do not generally involve in expansion as they are bound together by their mutual gravity. Objects which are farther from one another in space move faster, while those which are closer to each other will move at slower speed. This can make them to reach other objects. Likewise galaxies may collide and form new galaxies. Galaxies meet and fuse in a long period of time to form a large galaxy. In terms of our time scale, when two galaxies collide, nothing will happen to the celestial bodies existing in them.