For the first time, astronomers have found two giant clusters of galaxies that are just about to collide. This observation can be seen as a missing ‘piece of the puzzle’ in our understanding of the formation of structure in the universe, since large-scale structures—such as galaxies and clusters of galaxies—are thought to grow by collisions and mergers. The result was published in Nature Astronomy.
Source:
https://phys.org/news/2019-06-galaxy-clusters-caught.html