March 26, 2010 — Dark matter makes up the majority of mass in our universe. However, we cannot directly measure the stuff as it doesn't interact with electromagnetic radiation (i.e. it doesn't emit or reflect any light), but we can indirectly observe its presence.
In this beautiful multicolored Hubble Space Telescope image, the distribution of mostly dark matter has been calculated and mapped. Basically, the location and density of anything with mass has been plotted in a 3D representation of the cosmos.
But if the majority of matter (i.e. dark matter) cannot be seen, how did Hubble work out its location?
Hubble is making use of a characteristic of space-time as predicted by Einstein's theory of general relativity. Matter bends space-time — much like a bowling ball will warp a suspended rubber sheet because it's heavy — and as light travels through this bent space-time, the light's path will be deflected. This deflection can be directly observed.
For example, if a distant galaxy emits light in our direction, it may be diverted slightly in its otherwise straight path. Like a glass lens being placed in front of a lightbulb, the galactic light will distort from our viewpoint — the heavier the mass, the greater the distortion.
This distortion is known as "gravitational lensing" and it can be used as a tool to detect things like galaxies, black holes and, you guessed it, dark matter.
By combining the Hubble observations of gravitational lenses with spectroscopic red shift observations from telescopes on Earth, the 3D location of clumps of mass (dark matter, galaxies, black holes etc.) can be found. In this case, the white, cyan, and green regions are closer to Earth than those indicated in orange and red.
To produce this scene, astronomers used Hubble to survey over 446 000 galaxies, notching up over 1000 hours of observing time on the space telescope. This is the largest ever survey carried out by Hubble.
Red-shift data were collected for 194 000 galaxies in the same field of view by ground-based observatories. The further away the galaxy, the faster it's moving away from us (as the universe is expanding), making that galaxy appear "redder" than nearby galaxies.
Source: NASA/ESA Hubble Space Telescope