Distribution of Baryonic Matter: The density distribution of baryons in the modern Universe from a simulation by Cen & Ostriker (2006). Much of the mass of the missing baryons may lie within the filaments shown here connecting the higher density regions of matter. This image is the end result of a computer simulation that begins with smoothly distributed gas and follows the clumping of the gas that occurs because of gravity over the entire age of the Universe. The volume shown is 1000 times the volume of the Milky Way Galaxy. For more information about the simulation, see Cen, R. & Ostriker, J. P. 2006, ApJ, 650, 560.
Credit: Courtesy R. Cen and J.P. Ostriker

We have mathematical equations about the way the Universe evolved. We can start at very early times, set up a model, and then start the computer running and it will predict how things are going to come out. As I mentioned before, the baryonic content that has been detected in the nearby Universe is off by a factor of about two. This means that scientists are measuring 50% less baryonic matter than is predicted by the models. Now people are scratching around trying to find out where all of the missing baryons have gone. We think it is locked up in a pervasive hot gas that is very hard to see.

A paper published by two scientists asked, “Where have the baryons gone?” One suggestion was that it was in hot gas, which, for various reasons, we could not detect with our ultraviolet instruments. We recognized that with FUSE we could see five-times ionized oxygen. Now, five-times ionized oxygen is not in the hot gas I’m talking about; it’s more at the cool end of what I would call hot. We started a series of studies with FUSE and then our FUSE coworkers at the University of Colorado picked it up. I was just getting ready with NASA to dedicate FUSE in its last year of life to focus on this problem when the spacecraft died. As a result, we could never do the experiment, which was a great disappointment. But they did have enough data at the University of Colorado that they got some hints. The hints confirm that the idea that the missing baryons are locked up in the hot gas between galaxies is probably correct. But because we’re not measuring the main bulk of the gas, there’s a large margin of uncertainty in those measurements.

We’ve got a lot of tough challenges. I just provided an example -- where are the local baryons? Another one I’ll throw out is: “What is dark matter?” And an even harder one, and I don’t know how we’re going to solve this one: “What is this stuff called dark energy?”