Re: Let me get this straight ....
All matter in the universe is expanding, at an ever incresing rate i.e. getting further apart.
So how do 2 very big lumps of matter end up on a collision course?
To be technically accurate, you should say:
All On average the matter in the universe is expanding, at an ever increasing rate i.e. getting further apart.
It's not an absolute 'all' statement. Like with any other average, there are elements above and below the average that don't behave like the average. There are some regions were matter is moving apart faster than the average, some where it is moving more slowly than the average, and some where it is the opposite of that average, that is, it is moving together.
Designations like 'galaxy cluster', whether it's local group or cluster or supercluster refer to groupings of galaxies (or groups of groups) where, within that group, the gravitational attraction between the galaxies is stronger than the rate of the expansion of the universe. They are gravitationally bound to each other, therefore they are travelling along as a single group (on large scales), including orbiting each other, or towards each other and merging. As space stretches apart at <whatever the number is>km/s2, the gravitational attraction between the galaxies is equal to or greater than that expansion rate, keeping galaxies at the same approximate distance, or reducing the distance thus allowing for mergers.