It boggles the mind to hear that there are 100 billion galaxies in the universe. Of these, we think that the largest variety of galaxies are called giant ellipticals.
Giant ellipticals are relatively dull objects which feature nothing more than old stars slowly living out their lives. These galaxies contain giant complexes of stars with relatively little other material like extra gas that can form new stars. But we think these objects were not always so dull.
We think these galaxies were brightest about 10 billion years ago and have been fading ever since. To test this hypothesis, a group of us set out to find these putative intensely star-forming galaxies of the past.
We knew that about 10 billion years ago the high expected star formation rates would make these galaxies give us a veritable fireworks show. At the same time, a gigantic fireworks event on Earth produces so much dust that after a while we lose sight of the fireworks. Similarly, these galaxies would be putting on a giant continuous fireworks show for so long, about 100 million years continuously, that they should be literally buried in dust.
The bad news is that the dust prevents us from finding the galaxies. The good news is that dust also shines in far infrared colors. We could find candidates of these dust-enshrouded but otherwise hyperactive star forming galaxies by looking for far infrared colors in the sky consistent with being galaxies in the distant universe.
We found the brightest 200 such objects on the sky, and when we imaged them in detail we found not one galaxy but rather 10 or more individual intensely star-forming galaxies at each location. The massive galaxies of today appear to have been sociable and formed close together in space!