Scientists may not know what dark matter is, or where it comes from, but that doesn’t stop them from finding ways to image it. While some scientists still view the search for dark matter as nothing more than the latest fashionable physics snipe hunt, observational data continues to suggest that the invisible mass that supposedly glues our galaxies together is very much real. A group of astrophysicists hot on the trail of the mysterious substance have now managed to pick out a thread of the stuff bridging two distant galaxy clusters together.
Dark matter, as the name suggests, is hard to find. The only way to observe it is through gravitational lensing, an effect that happens when an object’s mass is so great that it distorts the light flowing around it like ripples on the surface of a pond. According to the LA Times, Joerg Dietrich and his team decided to look for the notoriously hard-to-spot stuff by searching for lensing around closely neighboring clusters of galaxies. The thought was that, because of the mass of the galaxies themselves, and their proximity to each other, it would be easier to spot any massive thread of dark matter connecting the two. When they turned their observations on a pair of galaxy clusters named Abel 222 and 223, they found that their hunch was correct.
According to simulations based on observable data, dark matter stretches like a cosmic web across the universe, binding galaxy clusters together. Up until now, no filament has been observed due to their relative size and length. Sometimes clusters of galaxies can exchange material between them by using these filaments of the dark matter web as a sort of gravitational conduit. X-Ray observations of Abel 222/223 showed that a small ribbon of gas and dust was connecting the two. Dietrich’s team ran simulations and found that not only was a dark filament present, but that it was also 58 light-years long.
If you are getting a bizarre feeling of déjà vu concerning this story, don’t freak out, you’re just a really big space geek. In March of this year, there was another dark matter announcement concerning a galaxy cluster named Abel 520. Hubble data showed that a collision of galaxy clusters had basically knocked the matter loose from its dark matter halo. The result was a large lump of dark matter sitting in its place with a disproportionately small number of galaxies left inside. This was a shock to astronomers since all current theories of dark matter theorize that it should cling to visible matter. In fact, the finding was so bizarre, that initial observations of it in 2007 were ignored as some sort of flaw in the data.
So yeah, we can spot it, but we still have no idea what it actually is.