I have been watching the history channel today and the programs where about the cosmos and there is a concept I don’t understand and maybe someone here can help. After the ‘big bang’ the universe is expanding and all the galaxies are moving away from us and away from each other, I get that. Another episode discussed galaxies and explained that many collided with each other including our own. So, may question is if the universe is expanding and galaxies moving away from each other how can some of them also collide? Has dark mass caused some of their paths to move out of a straight line? Just something I was wondering with no one I could ask personally. Thanks, Jack