CHAMPAIGN, Ill. — If you had a friend who looked like he weighed 150
pounds, but moved as though he were a ponderous 1.5 tons, you would
certainly wonder why.

Astronomers measuring the masses of galaxies and clusters of galaxies
are faced with a similar situation — there is not enough visible
matter to account for the gravitational motions of galaxies. Having
determined that nearly 95 percent of the mass in the universe
consists of an invisible, mysterious form of dark matter, they
naturally would like to know what the stuff is, where it came from,
and how it behaves.

Astronomy professors Brian Fields and Benjamin Wandelt, and graduate
students Richard Cyburt and Vasiliki Pavlidou, all at the University
of Illinois at Urbana-Champaign, have taken a new look at the nature
of this odd material. Their calculations, to be reported in the June
15 issue of Physical Review D, shed some light on the characteristics
of dark matter, and may lead to new theories about how structure
formed in the early universe.

“We endowed dark matter particles with certain microscopic properties,
and then we saw what the consequences would be for galaxies,” said
Wandelt, who also is a professor of physics at Illinois. “So, in a
sense, we looked at what inner space has to say about outer space.”

In the most popular dark matter model, Weakly Interacting Massive
Particles interact only through gravity. “But there is growing
evidence that suggests the structure of galaxies — the distribution
of visible matter embedded in large halos of dark matter — is not
what WIMP theories predict,” Wandelt said. “There doesn’t appear to
be as much mass piled up in galactic centers as we would expect if
dark matter was composed of WIMPs.”

To alleviate this problem, some astronomers have proposed a different
picture — one where dark matter particles can interact with one
another through forces other than gravity.

“Numerical simulations have shown that this self-interacting dark
matter does indeed predict halo cores in better agreement with the
observations,” Wandelt said. “But, if dark matter particles can
interact strongly with each other, then similar interactions might
be expected between them and ordinary baryonic matter as well.”

The Illinois team investigated the potential impacts such
interactions could have on big bang nucleosynthesis and on the
production of high-energy gamma rays.

Big bang nucleosynthesis is the process by which the primordial
elements, consisting mainly of deuterium (an isotope of hydrogen),
helium and lithium, were produced. In the standard scenario, dark
matter plays no role in the creation of these light elements.

“If dark matter can interact strongly with baryons, however, it is
possible that those interactions could destroy the newly forming
deuterium, and thereby delay the onset of nucleosynthesis,” Fields
said. “This would reduce the abundances of the light elements, of
course, which is contrary to what is observed.”

The researchers’ calculations show that the effect of
dark matter-baryon interactions is much smaller than the normal