There's something peculiar going on in the core of Andromeda, the Milky Way galaxy's nearby cousin.

Using NASA's Fermi Gamma Ray Observatory, astronomers have been able to detect a powerful source of gamma-ray radiation emanating from Andromeda and it could be the fingerprint of one of the biggest mysteries hanging over modern cosmology.

The gamma-ray emissions could be generated by the annihilation of dark matter particles and a similar signal has been detected coming from our own galaxy.

Dark matter constitutes nearly 85 percent of all mass in the universe (and normal matter — all visible stuff — therefore accounts for only 15 percent of all matter), but because it doesn't interact with the electromagnetic force, i.e. light, we cannot directly "see" it. We can, however, indirectly detect it by observing its gravitational effects on normal matter on galactic and cosmological scales.

The only problem is that we don't know what "it" is. But this new Fermi observation may bring us one step closer.

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Astrophysicists know that galaxies emit gamma-rays, but to have such a powerful gamma-ray signal coming from the center of Andromeda means something strange is going on.

"We expect dark matter to accumulate in the innermost regions of the Milky Way and other galaxies, which is why finding such a compact signal is very exciting," said astrophysicist Pierrick Martin, of the National Center for Scientific Research and the Research Institute in Astrophysics and Planetology in Toulouse, France.

It is thought that when dark matter particles, such as hypothesized weakly-interacting massive particles (or WIMPS), collide, they annihilate, releasing a sudden burst in energy. This energy is in the form of gamma-rays, so where Fermi sees an intense gamma-ray emitting region, it could indicate the location of a dense cloud of dark matter.

There could be another explanation, however. The researchers point out that a dense accumulation of pulsars could also generate an intense gamma-ray signal, but Andromeda is 2.5 million light-years away, so there's no way to resolve individual pulsar sources of gamma-rays.

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But as we know the Milky Way also generates gamma-ray radiation in its core, the researchers hope to compare the signal from Andromeda and the Milky Way to hopefully reveal if the signal is being caused by pulsars (that we can resolve in our own galaxy) or, more excitingly, dark matter particles annihilating in the cores of both galaxies.

"Our galaxy is so similar to Andromeda, it really helps us to be able to study it, because we can learn more about our galaxy and its formation," said research scientist Regina Caputo at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "It's like living in a world where there's no mirrors but you have a twin, and you can see everything physical about the twin."

Image: The gamma-ray excess (shown in yellow-white) at the heart of M31 hints at unexpected goings-on in the galaxy's central region. Credits: NASA/DOE/Fermi LAT Collaboration and Bill Schoening, Vanessa Harvey/REU program/NOAO/AURA/NSF

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