The location of the missing baryons is a big question. We think the 40% we have not directly observed is out there in intergalactic space, but in a much hotter phase. It's one to three million-degree gas. The way to find that is through X-rays. We need to look for absorption lines from the postulated hot gas using the very same techniques we use in ultraviolet absorption spectroscopy, but we need a bigger X-ray telescope. This is where I've gotten into X-ray astronomy. We've seen some hints of this absorption; we're starting to find some highly ionized oxygen out there between the galaxies. My bet is in 10 to 15 years, whenever the next big X-ray instrument comes, we'll be able to find the rest of the matter in a hotter phase. It's much too hot to be seen in the ultraviolet. That satellite, I don't know if you've already heard about it, is called IXO, International X-ray Observatory, and several of us here at Colorado are involved on the science team.
Some of these problems inevitably have to be solved with more than one wavelength. To find this missing matter, you need X-ray and ultraviolet, and ultimately optical to connect the gas to galaxies. You have this cosmic web, but that's just the gas. Where are the galaxies? It turns out the galaxies are located right along these filaments because they formed out of the gas, so the spiderweb created the galaxies. Therefore, you need an optical picture and an ultraviolet spectrum to understand where the galaxies are. It's a very multiwavelength task.