In 1933, Fritz Zwicky — a Swiss astronomer working at CalTech — had an amazing revelation. He realised that the amount of matter that we can see through our telescopes doesn't match the behaviour of the Universe. There had to be something else that we couldn't see. Something that accounts for an astonishing 83 percent of all existing matter but is invisible to us.
Or it was. Now, for the first time in history, a team of astronomers has observed these hidden tendrils that extend everywhere. Dark matter, the mysterious substance that appears to give the universe its structure, has finally been revealed.
The results of the study were published in Nature last Wednesday, going mostly unnoticed because of the announcement of the Higgs boson.
The research, lead by Jörg Dietrich, an astronomer at the University of Munich Observatory in Germany, observed the large-scale structure filaments intersections in which galaxy clusters occur:
[This marks] the first time we have observationally verified this very important theoretical prediction.
These dark tendrils have been impossible to observe until now because they are not dense enough. But Dietrich and his colleagues have been able to locate a filament that could be observed, a gigantic dark matter tendril that is 18 mega-parsecs long. It connects Abell 222 and Abell 223, two galaxy clusters located 2.7 billion light-years away from Earth, in the constellation Cetus.
The team was able to observe the dark matter not only because of the vast size of the filament, but because of its orientation: it is perpendicular to Earth, along our line of view. That made it more dense from our perspective, increasing the gravitational lensing that deforms the light of objects behind it. By observing this optical distortion over 40,000 background galaxies, they know that the mass in the filament is between 6.5 × 10^13 and 9.8 × 10^13 times the mass of the Sun.
They used data from the XMM-Newton spacecraft, processed with a new computer analysis technique that revealed the precise shape of the filament, thus showing the dark matter and confirming the theoretical model proposed by Zwicky at the beginning of the 20th century.
Until know, there has never been direct observation of this substance. You could have seen the effect in the 3D map of the universe below, which shows how galaxies are clumped along an apparently branched structure. However, this was just an indication of the existence of something invisible with enough gravitational power to make those shapes.
Both astronomers and physicists are very excited about this discovery. Astrophysicist Mark Bautz, at the Massachusetts Institute of Technology, thinks that "what's exciting is that in this unusual system we can map both dark matter and visible matter together and try to figure out how they connect and evolve along the filament." Alexandre Refregier, a cosmologist at the Swiss Federal Institute of Technology in Zurich, thinks that this observation will complement the work to find dark matter on Earth at laboratories like the Large Hadron Collider or Fermilab.
Slowly, every day, we are uncovering the secrets of the Universe one by one. And we'll continue to, until the Universe gets tired of us and sends us an giant asteroid to get rid of those pesky humans. [Nature and Nature via Boston Herald, Christian Science Monitor and NASA]