I have always been startled and fascinated by the sandlike abundance of galaxies sprinkled across the night sky. One of the most sensitive optical images ever made by human beings, the Hubble Ultra Deep Field, captures some 10,000 galaxies in an area about 1/100th the size of the full moon. Scaled up to the whole sky, such a density implies a total of 200 billion or so galaxies. And those are just the most luminous ones; the true number is probably much larger. How did all those galaxies come to be? This question inspired me to become an astronomer and has been the focus of my research career. Over the years my naive way of looking at galaxies has changed. To judge by their sheer numbers, nature appears to be quite good at producing galaxies. Not so. If you add up all the visible matter in galaxies today, you get only about a tenth of the total endowment created by the big bang. Where is the rest, and why did it not end up in galaxies? These are two of the biggest puzzles in astronomy today.
This missing matter is different from dark matter and dark energy. Those are substances of unknown composition that together amount to 96 percent of the total mass of the cosmos. In the case of numbers of galaxies, the trouble is with the 5 percent that was supposed to be well understood. This slice of the universe is normal matter, made of the same stuff as our bodies and everything around us—primarily baryons, the class of particle that includes protons and neutrons. For the majority of it to go missing is a mystery inside a mystery. Not only is most of the matter in the universe dark and unexplained, but of the small sliver that is normal, only a fraction is accounted for.