The map shows that the dark matter forms a filamentous 'skeleton' upon which visible matter congregates, eventually producing stars, Nature magazine reports. The composition of the dark matter is unclear but without it the universe could not exist. The dark matter is thought to act as a glue holding galaxies together.
"This is the first time that such a large-scale three-dimensional picture of dark matter has been produced, and it will allow cosmologists to probe deeper into the nature of this elusive matter," the report says.
The map also has a few puzzles within it. Some areas show clumps of dark matter that aren't accompanied by the bright features associated with conventional, visible material (made of baryonic matter), and vice versa.
"On the large scale the general picture is as expected, but there are some small-scale discrepancies," Richard Massey at the California Institute of Technology, Pasadena, and one of the team members who pieced together the map from hundreds of slightly overlapping images from the Hubble Space Telescope's Cosmic Evolution Survey was quoted as saying.
"The existence of large clumps of isolated dark matter and visible matter flies in the face of everything we know," says cosmologist Carlos Frenk of the University of Durham, UK.
The discrepancies, Nature says, could be a simple error resulting from the way the observations were made. But if they are real, says Massey, they will bring a huge shock.
Baryonic structures are expected to form only inside the dark-matter scaffold, says Massey adding, "There will need to be a lot of follow-up work before we really believe any individual discrepancies."
Massey, the report says, used a technique called gravitational lensing, whereby the pull from dark matter caught in between a star and the observing telescope alters the path of the light, and allows the presence of dark matter to be inferred.
Eric Linder of the University of California, Berkeley, who was not involved with the work, agrees that the map backs up the favoured theory that dark matter forms a scaffold on which galaxies form, Nature said.
He suggests possibilities for the more unusual spots in the map: one is that galaxies made of dark matter (dark galaxies) exist, but he thinks this is unlikely. Another possibility is that the discrepancies are errors in the data, which seem almost inevitable given that mapping the dark matter required a very sensitive measurement of an incredibly small signal.
There are, Nature says, plausible explanations for small areas of dark matter and visible matter existing in isolation.
Dark matter, if the clump is small enough, could have any accumulating visible matter blown out of it by a high-energy phenomenon such as a quasar or a supernova, for example.
"The collision of two galaxies could also blow an amount of visible matter out as a faint satellite galaxy that has no associated dark matter, suggests Frenk. But these theories can't explain the large features visible on the COSMOS map, he adds.
