The expansion of the universe is at the grandest of all scales... a scale that is MUCH larger than the average distance between stars and between galaxies. Just like you can clap your hands and merge them in an expanding universe, the same thing can happen for galaxies, this is because the SCALE of the universe is MUCH, MUCH, MUCH larger than a single galaxy.

Great question! When we say that the universe is expanding, we really mean that the universe overall is expanding: that doesn't mean that everything is moving away from everything else (for example, you're not expanding away from your friends all the time!). This is because on short distances, there are forces than can counteract the expansion of the universe. For example, the electromagnetic forces that keep the molecules in our bodies together are WAY stronger than the force that's expanding the universe, so our bodies aren't expanding. Similarly, the gravitational pull between the sun and the Earth is also much stronger than the force expanding the universe, so the Earth stays put in its orbit around the sun. It's the same thing with galaxies: the gravitational pull between nearby galaxies is strong enough to make them collide. It's only when galaxies are really far apart that the universe's expansion is able to pull them farther away from each other.

Good question!
The answer is that the universe is expanding *in general*, but portions of the universe that are bound to each-other by gravity are not expanding. This includes planets, stars, star systems, star clusters, galaxies, and even galaxy clusters. Because galaxy clusters are themselves bound by gravity, the galaxies in those clusters can and do collide with other galaxies from the same cluster.

Galaxies from different clusters do not collide. At that scale, the universe is expanding.