See Inside May 2011

The Lost Galaxies

By the latest estimate, the observable universe contains 200 billion galaxies. Astronomers wonder: Why so few?

Graphic by James E. Geach and Rob Crain

I have always been startled and fascinated by the sandlike abundance of galaxies sprinkled across the night sky. The most sensitive optical image 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.

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