They can be 20 times the Milky Way's size and contain more than a trillion stars. Such jumbo galaxies probably formed when two or more spirals, like the Milky Way, merged to form a single galaxy. One bit of evidence supporting the merger theory is the large number of ellipticals in dense clusters of galaxies, where mergers must be common. Two giant ellipticals dominate the center of the densely packed Coma Cluster, for example. And the heart of the Virgo cluster contains three giant ellipticals that each span about two million light-years.
Mergers can take anywhere from a few hundred million to a few billion years to complete.
They can trigger intense bursts of new star formation, and even create gigantic black holes.