Ok, Keith, there is the Warm-Hot Intergalactic medium (WHIM), I don't know much about this but have been looking it up. This is gas at maybe 100,000 degrees (not as hot as that in clusters of galaxies). It can be found in X-rays by a satellite such as Chandra, or detected by its absorption of light from quasars behind it (by the Cosmic Origins Spectrograph on Hubble). I am not up with this stuff, but will read up more, but it seems to be that it makes up maybe half or more of the non-dark matter in the universe. There was a big press release from Chandra about 4 years ago saying that they had found loads of it.