In the throes of formation, a galaxy like our Milky Way pulls in dense, cold gas (red streams) and also ejects hot gas (blue streams) back into intergalactic space. The galaxy ends up with only a small fraction of the raw material. The author and his colleagues generated this image using a state-of-the-art cosmological simulation code. Image: Graphic by James E. Geach and Rob Crain
- Forget dark matter: even the supposedly normal matter of the universe is mysterious enough. Why does only a small fraction of it reside in galaxies? Where did the rest go?
- The current best guess is that the bulk of the normal matter is trapped in giant gaseous filaments. This so-called warm-hot intergalactic medium, or WHIM, is hard to detect directly.
- Galaxy formation is evidently rather inefficient. As material falls into a galaxy, the galaxy tends to shoot much of it straight back out again—a process known as feedback.
- The atoms in your body have probably been cycled through intergalactic space. Indeed, galaxies and their contents are not fixed structures but the bright tips of a wider sea of gas.
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.
This article was originally published with the title The Lost Galaxies.