The Framework of the Universe
Dark matter is the skeleton of the universe. This framework, around which the usual visible matter is arranged, is itself completely invisible. But that's no reason not to create a detailed overview in 3-D anyway, found resourceful cartographers of the universe.
More than eighty percent of the total mass of the universe consists of dark matter. It does not emit any electromagnetic radiation such as visible light. It can only interact with normal, so-called "baryonic" matter or with light through gravitation. "Only" - but precisely this property was sufficient for an international team of scientists to draw a three-dimensional distribution map of dark matter.
The data the researchers used as a visual baseline came from the Hubble Space Telescope. As part of COSMOS, the "Cosmic Evolution Survey", Hubble mapped a relatively large section of the universe with high resolution: 1.6 degrees squared. This section, the COSMOS field, has eight times the area of the full moon and is thus the largest section of the universe measured in this way to date. This has resulted in an extensive collection of very detailed images of half a million galaxies.
With this data, however, the astrophysicists had more in their hands than just pretty pictures - conversely, they were also able to determine the distribution of dark matter from the shape of the galaxies. The method they used is based on the "weak gravitational lensing effect". This effect can be explained by a simple analogy: when light passes through a frosted glass pane with a roughened surface, such as in bathroom windows, it casts a characteristic pattern on the wall. This pattern provides information about how the surface of the glass is structured - or, in the case of the universe map, how the dark matter must be distributed, where the rays of light are not deflected by frosted glass but by the gravity of the invisible masses.
Of course, baryonic matter also affects light, for example a star or a galaxy. In contrast to dark matter, the stars of the galaxies are visible, and the scientists were therefore able to deduce their mass from their color and distance. Led by Günther Hasinger and his group at the Max Planck Institute for Extraterrestrial Physics, the COSMOS field was also surveyed using ESA's XMM Newton telescope. This telescope measures X-rays from hot gas in the Universe, the baryonic matter that clumps in places where dark matter is particularly abundant. The resulting X-ray image was of particular value to the mapping project: it confirmed the distribution of baryonic matter completely independently of gravitational lensing measurements, thereby helping to calibrate the method used.
All in all, dark matter presents itself as a network that is distributed unevenly across the universe, as exemplified by the recently published universe section with around 500,000 galaxies. In some places, dark matter clumps very densely, and this is where the galaxies and hot gas of normal matter, made up of baryon subatomic particles, are always found.
Obviously, baryonic matter follows dark matter - a confirmation of the standard theory that has been valid so far regarding its role as the framework of the universe. According to the ideas of the astrophysicists, it was originally evenly distributed, but then became more concentrated in some areas. Here, the pull of the stronger gravity then ensures that visible matter clumps together: stars, galaxies and entire galaxy clusters are formed. © Max Planck Society
The Max Planck Society (MPG) is a basic research institution funded primarily by the federal and state governments. It operates around eighty Max Planck Institutes.