Cosmic accounting

A couple of people have asked me about a new discovery by Chandra (orbiting x-ray observatory) of lots of mass in distant gas clouds (NASA release).

What has been discovered is more baryons (ordinary stuff, like protons and neutrons), not the exotic dark matter. There are strong limits from nucleosynthesis (synthesis of light nuclei early in the hot big bang) that constrain the baryon fraction of the total energy density to be about 5%. The dark matter (non-baryonic matter of unknown type clumped around galaxies or clusters of galaxies) is about 25% of the total and dark energy (weird stuff with negative pressure) is the remaining 70% or so.

About half of the expected 5% baryon fraction had been accounted for in galaxies, and these huge gas clouds may be the rest of it. We're still in the dark about dark energy and dark matter, though.

One mystery puzzling theorists is why there are roughly (within an order of magnitude or so) equal amounts of these 3 types of stuff. As the universe expands, the energy density of dark energy changes very slowly compared to baryons or dark matter. Unless we are living at a very special epoch, one would expect the densities of these things to differ by many orders of magnitude. (For example, when the universe was 10 times younger the DE fraction was tiny, and when it is ten times older the DE fraction will be nearly 100%.)