The cosmic microwave background radiation (CMB) provides a glimpse at the baby Universe, shortly after its birth in the Big Bang. By mapping out the patterns and irregularities in this “baby picture,” researchers can get a rough idea of how much ordinary (baryonic, not dark) matter there should be in the present Universe, with respect to the dark.
The estimate they’ve obtained this way suggests that baryonic matter should account for about five percent of the stuff (energy content) in the Universe. But looking at the nearby Universe, researchers find a baryonic matter distribution that’s about half of this estimate.
One possible solution to this discrepancy is that the local Universe has lower density than elsewhere and that the filaments of the cosmic web have a higher percentage of baryonic matter. This scenario is suggested by computer simulations. Essentially, the idea is that this extra matter would be “warm-hot,” which in space just means the particles are moving around very quickly.