The center of the Milky Way galaxy is mysteriously glowing, and of course, there is a whole host of stars there, along with a black hole 4 million times the mass of the sun. But, with the light subtracted from all of that, the region remains inundated with a mysterious excess gamma radiation.
It's called the GeV Excess (GCE) Galactic Center, and scientists have puzzled since its discovery by physicists Lisa Godineau and Dan Hopper, in 2009. In data from NASA's Fermi Telescope, they found excess gamma radiation, some of the most energetic light in the universe.
Physicist Mattia de Moreau, of the National Institute of Nuclear Physics in Italy, said his analysis points to dark matter as the culprit in GCE (this was first put forward as an explanation by Godino and Huber).
It is known that there is a mysterious mass responsible for the effects of gravity, which cannot be explained by things that we can discover directly - natural matter such as stars, dust, gas and galaxies.
For example, galaxies rotate much faster than they should if their gravity is affected by natural matter. Gravitational lensing, the bending of space-time around massive objects - is much stronger than it should be. Everything that creates this extra oomph is beyond our ability to directly reveal.
And we only know dark matter through the effect of gravity on other objects, and there is a lot of it out there. Nearly 80% of all matter in the universe is dark matter, although we cannot see a scrap of it.
The researchers suggested that if certain dark matter particles called WIMPS (weakly interacting massive particles) and their antiparticles collide, they would annihilate each other and explode in a shower of other particles, including gamma ray photons. This explanation, they said, fits surprisingly with the data. Other physicists weren't convinced, with one even describing the explanation as wobbly.
And in 2018, another team of scientists suggested that very old dead stars called pulsars could cause overflow. This makes sense, because the galactic center is so crowded, dusty, and lively - it would be very easy to miss one or more stars.
Recent studies also found that the GCE distribution is not smooth - as you would expect from the dark matter annihilation but rather lumpy and speckled, which the pulsar team interpreted as compatible with point sources, such as stars.
Then another team came and decided that thin gamma radiation could be produced by dark matter, which brought it back to the table. However, more researchers have created a series of comprehensive models of the galactic center with the annihilation of dark matter using an array of masses across the most researched systems. And they found that WIMPs were unlikely to be the cause of GCE.
De Mauro's study compares data from the Fermi telescope over the past 11 years, with measurements of other astronomical anomalies recorded by Pamela's cosmic ray detector aboard Resurs-DK No. 1 and the Alpha Magnetic Spectrometer aboard the International Space Station.
In particular, the study uses the largest set of data from Fermi, collected over the past year, and reduces the uncertainty caused by background radiation. De Mauro said this provided information about the spatial distribution of the GCE, which could help rule out the various interpretations.
He explained, if, for example, the excess is a result of the interaction between cosmic rays and atoms, then we would expect to observe their greater spatial distribution at lower energies, and their lower propagation at higher energies due to the diffusion of cosmic particles. My study, on the other hand, confirms how the distribution is. The spatial increase does not change as a function of energy.
This had not been observed before, and could be explained by dark matter, because we think dark matter particles should have similar energies. The analysis clearly shows that the increase in gamma rays is concentrated in the center of the galaxy, which is exactly what we would expect to find in the heart of the Milky Way if dark matter was in fact a new type of particle.
In a second preprint paper, de Moreau and his colleague Martin Wolfgang Winkler of Stockholm University in Sweden attempted to look for excess gamma-rays in nearby dwarf globular galaxies. They found nothing, but this empty discovery made it possible to impose restrictions on the mass of the dark matter particle. They said these limitations are compatible with the GCE.
Does this mean that dark matter causes GCE? No - at the same time, we cannot say that dark matter does not cause it either. And basically, everything is as baffling as before, and we're going to need some great science (and a lot of notes, probably) in order to get to the bottom of it.
De Mauro's research has been published in Physical Review D, and the second paper is available at arXiv.