Dark matter is a hypothesized material that pervades the universe but
does not interact with light in a way that we can perceive. Current
estimates rate dark matter as being roughly five times as prevalent as
ordinary matter—the atoms and molecules that make up the familiar
physical world. To date, dark matter has been observed only indirectly
via its gravitational effects, but its true nature remains a mystery.

A paper posted to the physics preprint Web site arXiv.org on October
26 and submitted to the Astrophysical Journal points to a possible
signature of dark matter in the Milky Way, although the study's
authors are careful to keep their observations empirical and table
such speculation—for the moment, at least.

In 2003 Douglas Finkbeiner, an astronomer at the Harvard–Smithsonian
Center for Astrophysics, noticed a diffuse haze toward the center of
the galaxy in microwave data collected by NASA's Wilkinson Microwave
Anisotropy Probe (WMAP). There are only a handful of processes that
yield microwaves in the interstellar medium, Finkbeiner explains, and
when he subtracted templates for those processes from the WMAP data,
something curious remained. "If our model [for microwave production]
were correct, we would have random noise left over," Finkbeiner says.
"Instead, we see a pattern, an excess of microwaves in the inner
galaxy."

He and his colleagues figured that the microwaves were synchrotron
emission: photons emitted by electrons accelerated by the galaxy's
magnetic field. But the energy spectrum of the electrons was not
readily accounted for by conventional sources in the inner galaxy—for
instance, electrons originating from supernovae explosions. So, a
popular model for dark matter, in which the dark particles would
annihilate each other on contact in a burst of observable particles,
including electrons, seemed instead to fit the bill.

The high-energy electrons suspected as the progenitors of the WMAP
haze should produce a similar haze in the gamma-ray regime, Finkbeiner
and his colleagues predicted. "We would expect that those same
electrons that are spiraling around the galactic magnetic field will
once in a while hit a photon coming from a star or something like
that," says Gregory Dobler, a postdoctoral researcher at the Kavli
Institute for Theoretical Physics at the University of California,
Santa Barbara, and lead author of the new arXiv study. Such electrons
can bump up optical or infrared photons to gamma-ray energies, Dobler
notes, where they could be detected by NASA's Fermi Gamma-Ray Space
Telescope.

When Fermi's first year of data went public in August, Dobler,
Finkbeiner and their colleagues set to finding out whether the gamma-
ray map of the galaxy indeed featured a haze akin to that seen by WMAP
in the microwave. Sure enough, it did. "I had originally thought that
it would be difficult to peel away all of the other types of things
that make gamma rays in the galaxy, but I was kind of shocked," Dobler
says. "It's almost jumping right out at you."

In the new paper Dobler and his colleagues describe the Fermi gamma-
ray haze and make the claim that it confirms the synchrotron origin of
the WMAP microwave haze. And as with the microwave haze, the authors
argue that the electrons responsible for the gamma-ray haze appear to
originate from an unknown astrophysical process.

Sean Carroll, a theoretical physicist at the California Institute of
Technology who did not contribute to this haze research, calls the
astrophysical claim that the hazes spied by Fermi and WMAP point to a
population of high-energy electrons swirling around the inner galaxy
"very reasonable." If the claim is true, he adds, "then the question
is, 'Where do these electrons come from?' And one very plausible
origin would be dark matter particles either decaying or annihilating
into each other and creating a spray of particles."

Finkbeiner points out that he and his colleagues do not speculate in
the new paper on a potential dark matter source for the haze-forming
electrons. But both he and Dobler acknowledge moving in that
direction. "We are absolutely in the process of exploring the Fermi
haze in the context of dark matter physics," Dobler says.

But the implications of the gamma-ray and microwave hazes may remain
open to challenges that the hazes themselves are as significant as
they appear. "Everyone recognizes that it's a very tricky thing to
look at all this messy radiation coming from the center of the galaxy
and really to say that we've removed all the stuff we understand and
are left with something we don't," Carroll notes.

Dark matter is a hypothesized material that pervades the universe but
does not interact with light in a way that we can perceive. Current
estimates rate dark matter as being roughly five times as prevalent as
ordinary matter—the atoms and molecules that make up the familiar
physical world. To date, dark matter has been observed only indirectly
via its gravitational effects, but its true nature remains a mystery.

A paper posted to the physics preprint Web site arXiv.org on October
26 and submitted to the Astrophysical Journal points to a possible
signature of dark matter in the Milky Way, although the study's
authors are careful to keep their observations empirical and table
such speculation—for the moment, at least.

In 2003 Douglas Finkbeiner, an astronomer at the Harvard–Smithsonian
Center for Astrophysics, noticed a diffuse haze toward the center of
the galaxy in microwave data collected by NASA's Wilkinson Microwave
Anisotropy Probe (WMAP). There are only a handful of processes that
yield microwaves in the interstellar medium, Finkbeiner explains, and
when he subtracted templates for those processes from the WMAP data,
something curious remained. "If our model [for microwave production]
were correct, we would have random noise left over," Finkbeiner says.
"Instead, we see a pattern, an excess of microwaves in the inner
galaxy."

He and his colleagues figured that the microwaves were synchrotron
emission: photons emitted by electrons accelerated by the galaxy's
magnetic field. But the energy spectrum of the electrons was not
readily accounted for by conventional sources in the inner galaxy—for
instance, electrons originating from supernovae explosions. So, a
popular model for dark matter, in which the dark particles would
annihilate each other on contact in a burst of observable particles,
including electrons, seemed instead to fit the bill.

The high-energy electrons suspected as the progenitors of the WMAP
haze should produce a similar haze in the gamma-ray regime, Finkbeiner
and his colleagues predicted. "We would expect that those same
electrons that are spiraling around the galactic magnetic field will
once in a while hit a photon coming from a star or something like
that," says Gregory Dobler, a postdoctoral researcher at the Kavli
Institute for Theoretical Physics at the University of California,
Santa Barbara, and lead author of the new arXiv study. Such electrons
can bump up optical or infrared photons to gamma-ray energies, Dobler
notes, where they could be detected by NASA's Fermi Gamma-Ray Space
Telescope.

When Fermi's first year of data went public in August, Dobler,
Finkbeiner and their colleagues set to finding out whether the gamma-
ray map of the galaxy indeed featured a haze akin to that seen by WMAP
in the microwave. Sure enough, it did. "I had originally thought that
it would be difficult to peel away all of the other types of things
that make gamma rays in the galaxy, but I was kind of shocked," Dobler
says. "It's almost jumping right out at you."

In the new paper Dobler and his colleagues describe the Fermi gamma-
ray haze and make the claim that it confirms the synchrotron origin of
the WMAP microwave haze. And as with the microwave haze, the authors
argue that the electrons responsible for the gamma-ray haze appear to
originate from an unknown astrophysical process.

Sean Carroll, a theoretical physicist at the California Institute of
Technology who did not contribute to this haze research, calls the
astrophysical claim that the hazes spied by Fermi and WMAP point to a
population of high-energy electrons swirling around the inner galaxy
"very reasonable." If the claim is true, he adds, "then the question
is, 'Where do these electrons come from?' And one very plausible
origin would be dark matter particles either decaying or annihilating
into each other and creating a spray of particles."

Finkbeiner points out that he and his colleagues do not speculate in
the new paper on a potential dark matter source for the haze-forming
electrons. But both he and Dobler acknowledge moving in that
direction. "We are absolutely in the process of exploring the Fermi
haze in the context of dark matter physics," Dobler says.

But the implications of the gamma-ray and microwave hazes may remain
open to challenges that the hazes themselves are as significant as
they appear. "Everyone recognizes that it's a very tricky thing to
look at all this messy radiation coming from the center of the galaxy
and really to say that we've removed all the stuff we understand and
are left with something we don't," Carroll notes.

And indeed, some researchers are not yet convinced that the hazes
represent anything out of the ordinary. Astrophysicist Charles Bennett
of Johns Hopkins University, principal investigator on the WMAP
mission, says that his team has not viewed the haze as an excess of
emission from the galactic center but as part of the natural
astrophysical variation of synchrotron radiation. He points to a
recent paper that seeks to explain the electrons by conventional
astrophysical mechanisms, including massive supernovae, that do not
require the influence of dark matter. "The observations, so far, can
be explained either way," Bennett says, noting that the predictions of
each model need to be checked against one another. "In the meantime,
there is no particular evidence favoring what might be viewed as the
more exotic explanation—dark matter decay."