Answer to Question 1
The pull of gravity works to slow the expansion rate of the universe. Therefore, in dense regions such as galaxies and clusters of galaxies, gravity is able to overcome the expansion and keep these objects from expanding. However, on larger scales, where the average density is lower, gravity does not have as strong an effect, and therefore the expansion continues between clusters of galaxies.
Answer to Question 2
Space itself is believed to have a frothy bubble-like structure, with superclusters of galaxies forming along the surface of a bubble in a filamentary pattern. Large voids are found between the filaments.