Shedding Light on Dark Matter

Dark matter has been perhaps the most famous and interesting of astronomical mysteries. Amazingly enough, even with the intense scrutiny done by the HESS group and the Fermi telescope group lately, it appears it will remain that way for the time being.

Two papers just published by the aforementioned groups cite studies of high-energy electrons and their anti-matter counterparts, positrons. Previously thought to necessarily be dark matter, the two papers now claim that they could derive from the annihilation of dark matter particles in the halo of our galaxy. This would mean that they would only be indirect evidence of the dark matter the astronomical community is so hoping to finally directly observe.

Dark matter is so-called because it has traditionally been impossible to observe directly. Instead, we know it exists because of the evidence we see. The most famous evidence, perhaps, is the rotation curve of our own galaxy. By studying how our galaxy rotates, physicists long ago discovered that the gravity evidenced by the rotation curve did not match up with the matter we knew existed and had observed. The only explanation had to be that there was some other matter there, which would explain the gravitational force experienced by our galaxy, but which we could not see.

Dark matter could, by definition, be anything which does not emit its own light. Planets, meteors, dust, particles, and anti-particles were initially all in the running. Physicists even humorously nicknamed the top two initial candidates MACHOs (Massive Compact Halo Objects) and WIMPs (Weakly Interacting Massive Particles). While still entirely up in the air, most physicists agree that dark matter is more than likely composed of an excess of particles.

Last summer, the astronomical community was abuzz about a picture published by the PAMELA detector, a satellite-borne telescope, which seemed to show an excess of high-energy positrons. Some physicists analyzed this data, coming to the conclusion that this excess must have been coming from dark matter annihilation. As a corollary, the South-Pole-based balloon experiment, ATIC, published results citing an excess in high-energy electrons, which were also suggested to come from dark matter annihilation.

The results of this analysis seemed to indicate, not only that we were finally finding a way to directly observe dark matter, but also that we knew what it was. All signs seemed to point to an end to the mystery: Dark matter was simply an excess of particles.

However, the cosmic ray detector that started the excitement, PAMELA, has now squelched it. A few weeks before the publication of the ATIC results, PAMELA reported an absence of an excess of anti-protons and protons in the cosmic ray flux. This excess would necessarily correlate to dark matter annihilating, and the lack thereof meant that the previously-reported excesses could not be due to dark matter annihilation.