The WHIM is actually extremely non-dense: 1-10 particles per cubic meter. It turns out that matter can be extremely hot without being dense at all, as long as there's nothing to cool the matter off, as is the case in the space between galaxies.
Matter is easy to see when it emits light (like stars) or absorbs light (like uncharged matter). But charged plasma does neither, so it's hard to detect. Fortunately, the WHIM has a prism-like effect, where it smears out light travelling through it, so we can detect it.
Matter is easy to see when it emits light (like stars) or absorbs light (like uncharged matter). But charged plasma does neither, so it's hard to detect. Fortunately, the WHIM has a prism-like effect, where it smears out light travelling through it, so we can detect it.