We are told that the universe is not a collection of galaxies sitting in space, all moving away from a central point. Instead, a more appropriate analogy is to think of the universe as a giant blob of dough with raisins spread throughout it (the raisins represent galaxies; the dough represents space). When the dough is placed in an oven, it begins to expand, or, more accurately, to stretch, keeping the same proportions as it had before but with all the distances between galaxies getting bigger as time goes on.
I have a query, is if this is the case why are certain galaxies colliding?