Giant Galaxies

One observes that rich clusters of galaxies often have a few giant elliptical galaxies near their center, and there is some evidence that these galaxies may have grown large by eating their neighbors. Such galactic cannibalism was far more common in the ancient past, and large galaxies may have grown to their current size by consuming their neighbors.

If this picture is correct, the ultimate building blocks of large-scale structure may have been million-solar-mass clumps that theorist believe were abundant when the Universe was only a few million years old.