There is a hidden universe out there, which is intertwined with the universe that we can see. This secret world is made of dark matter: a type of particle that has mass, but we can't see it. In spite of this, there are still ways of detecting it, and scientists have started using this technique to map out the concealed part of our universe.
The map has been made using a new wide-field camera on the Subaru Telescope in Hawaii. It is called Hyper Suprime-Cam and it's finished mapping a 2.3-square-degree section of the universe around the Cancer constellation. It has revealed that there are nine large concentrations of dark matter in this region. Each concentration has an enormous mass, comparable to that of a galaxy cluster. The research was published in the Astrophysical Journal.
It might seem a bit crazy to make a map of something that we can't even see or detect, but, fortunately, there is a way to infer the existence of dark matter. Einstein's theory of general relativity predicts that objects with mass warp space-time around them (which is how planets stay in orbit around their central star). Surprisingly, rays of light are influenced by these warped regions of space and bend their path to follow the contours of space-time around massive objects. This can feel counterintuitive as light is typically described as moving in straight lines. This prediction is fine for experiments on Earth, but bending light is prevalent in space.
When peering into the sky, there are occasional galaxy blurs and inconsistencies that can be seen over the universe, like the ones in the picture below. By mapping the deviations of these glimmers back to their source, scientists can deduce how much unseen mass would be responsible for the light deviation. This mass is dark matter.
Subtle, circular smudges in the night sky caused by gravitational lensing. NASA.
The first results from the camera are promising; the next stage is to make the map larger to see the distribution of dark matter in a thousand square degrees of sky, a huge goal.
The map itself is an important tool in the quest to understand the nature of dark matter and the properties of dark energy. There are dark matter maps that have been made in the past, but this one is aiming for a new level of detail.