First, a bit of background to this ongoing story. Dark matter likely makes up around 83 percent of all matter in the universe. But scientists thus far have not been able to observe it directly, because it interacts so weakly with ordinary matter; we only infer its existence from detecting their gravitational fields.
A physicist named Fritz Zwicky first noticed this phenomenon in 1933 when he concluded that galaxies in the Coma cluster were moving so quickly that they should be able to escape from the cluster if visible mass was the only thing contributing to the cluster's gravitational pull. Since the cluster hadn't flown apart, he proposed the existence of "dark matter" to account for the observational data.
In the 1960s, Vera Rubin and Kent Ford confirmed Zwicky's theory when their spectral analysis revealed that the outer stars in selected spiral galaxies were orbiting just as quickly as those at the center.
The visible matter wasn't sufficient to account for this; the spiral galaxy should be flying apart. Clearly, there had to be some kind of hidden "dark" mass adding to the galaxy's gravitational influence.
Physicists have been trying to directly observe dark matter ever since. We can, however, indirectly observe dark matter through gravitational lensing. That extra mass exerts a gravitational force on the space-time surrounding it. As light travels from distant galaxies, it will be bent around gravitational distortions in space-time - much like the paths of marbles rolling across a bent sheet of plastic - being caused by the dense regions of dark matter.
But gravitational lensing is a small effect, and difficult to detect for individual galaxies. That's why the SDSS data is so important: now scientists have images of millions of galaxies at their disposal. In 2010, an international research group used that data to determine the distribution of projected matter density from the center of galaxies out to a hundred million light years.
Just last month, an international team of astronomers announced that they had analyzed light from 10 million galaxies in four different regions of the sky and built the largest dark matter map created to date: an intricate cosmic web of dark matter and galaxies one billion light-years across.
Even NASA's Hubble Space Telescope has a mission dedicated to mapping dark matter, specifically within galaxy clusters. It's called the Cluster Lensing And Supernova survey with Hubble (CLASH), and so far it has measured six of 25 target galaxy clusters.
The new computer simulations of the large-scale cosmic structure by researchers at the University of Tokyo's Institute for the Physics and Mathematics of the Universe (IPMU) and Nagoya University shed further light on exactly where dark matter lurks in the universe. They build on the 2010 results to break down the distribution of each type of matter, not just the projected matter distribution as a whole.
Among the most interesting findings is that galaxies don't have well-defined "edges," per se. Instead they have sweeping "outskirts" of dark matter, extending out to nearby galaxies like a bride's flowing train - and even out into intergalactic space. Intergalactic space, they conclude, is far from empty: it's teeming with dark matter.
Granted, this is a computer simulation, but it is based on real, observational data from the SDSS. Stay tuned to see what further insights we can learn about dark matter as other projects complete their analyses. Our "map" of dark matter in the universe will keep getting better and better.
Images: Top: The simulated distribution of dark matter through the cosmos. Middle: The effect of gravitational lensing around a galaxy. Credit: Joerg Colberg, Ryan Scranton, Robert Lupton, SDSS