The Universe is a big place, and there are a lot of things "out there" that
astronomers can't identify. NASA's Fermi space telescope has just found about
600 examples. The Fermi team recently released the second catalog of gamma-ray
sources detected by their satellite's Large Area Telescope. Of the 1873 sources
found, nearly one-third are complete mysteries. No one knows what they are.
"Fermi sees gamma rays coming from directions in the sky where there are no
obvious objects likely to produce this kind of radiation," says David Thompson,
Fermi's Deputy Project Scientist. "It's a puzzle."
Gamma rays are by their very nature heralds of great energy and violence. They
are a super-energetic form of light produced by sources such as black holes and
massive exploding stars. Gamma-rays are so energetic that ordinary lenses and
mirrors do not work. As a result, gamma-ray telescopes can't always get a sharp
enough focus to determine exactly where the sources are. And therein lies the
For two thirds of the new catalog's sources the Fermi scientists can, with at
least reasonable certainty, locate a known gamma ray-producing object such as a
pulsar or blazar. But the remaining third, the "mystery sources", have the
researchers stumped, at least for now. And they are the most tantalizing.
"Some of the mystery sources could be clouds of dark matter something that's
never been seen before."
About 85% of the gravitational mass of the universe is dark matter. The stuff we
see makes up the rest.
Dark matter is something that pulls on things with the force of its gravity but
can't be detected in any other way. It doesn't shine, doesn't emit or scatter
light - hence the name "dark" matter.
Astronomers are not able to detect dark matter directly using optical or radio
telescopes. But dark matter just might shine in gamma rays.
"We've been using Fermi to search for dark matter for a long time," says the
principal investigator for the Large Area Telescope, Peter Michelson of Stanford
Some researchers believe that when two dark matter antiparticles bump into each
other, they will annihilate, producing gamma rays. Concentrated clouds of dark
matter could form a gamma ray source at specific wavelengths detectable by
"If we see a bump in the gamma-ray spectrum, a narrow spectral line at high
energies corresponding to the energy of the annihilating particles - we could be
the first to 'apprehend' dark matter," says Michelson.
So far, however, the team doesn't have enough data on the mystery sources to
confirm or rule out the dark matter hypothesis. Other possibilities include
undiscovered pulsars, supernova remnants, and colliding clusters of galaxies.
The scientists plan to continue observing until they have some answers.
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