First, there was the dark matter that we could not see. Now, large chunks of ordinary matter in the Universe have also gone missing.

According to standard theories of physics, only 17 percent of all the matter in the Universe is made up of what we call ‘baryonic matter’: protons, neutrons, electrons, and all the things that make up you and me. The remaining 83 percent is dark matter, an unknown substance that we can only detect via its gravitational attraction. However, new research published in the online edition of the Astrophysical Journal claims to show that galaxies are falling far short of this ratio of ordinary matter.

“One would expect galaxies and clusters of galaxies to be made of the same stuff as the Universe as a whole, so if you make an accounting of the normal matter in each object, and its total mass, you ought to get the same 17 percent fraction,” says Professor Stacy McGaugh of the University of Maryland, who presented new results at the meeting of the American Astronomical Society in Washington, DC today. “However, our work shows that individual objects have less ordinary matter, relative to dark matter, than you would expect from the cosmic mix – sometimes a lot less!”

McGaugh and his team found that the smallest galaxies contain the lowest fraction of ordinary matter, as little as 0.2 percent. In other words, 99.8 percent of these galaxies is made from dark matter. This actually matches observations of small satellite galaxies in orbit around the Milky Way (for example, see the story of the dwarf galaxy Segue 1). In contrast, the largest galaxy clusters appear to contain up to 14 percent ordinary matter – as close to 17 percent as anywhere in the Universe. In essence, it all seems to depend on scale. “Put another way,” says McGaugh, “the smallest galaxies are very dark matter dominated.”

So the question is, where has the rest of the baryonic matter gone? “The short answer is, we don’t know,” admits McGaugh. “There are various lines of speculation, most of which are either easily dismissed or are un-testable. For now, this is a problem without an obvious solution.”

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