Dark matter is not the stars and planets that we see and it is not in the form of dark clouds of normal matter - baryons - because we would be able to detect baryonic clouds by their absorption of radiation passing through them. Dark matter is not antimatter because we don't see the unique gamma rays that are produced when antimatter annihilates with matter and we can rule out large galaxy-sized black holes on the basis of how many gravitational lenses we see. High concentrations of matter bend light passing near them from objects further away, but we do not see enough lensing events for dark matter contribution.
The most common view is that dark matter is not baryonic at all, but that is made up of other, more exotic particles like axions or WIMPS (Weakly Interacting Massive Particles) which are, counterintuitively, their own antiparticles.
Now researchers say a pair of galaxy clusters, Abell 222 and Abel 223, 2.7 billion light-years away and each containing thousands of galaxies. has shown a giant string of invisible dark matter.
The idea is not new, of course but the people behind the current study contend other efforts (such as 'Weak lensing study of dark matter filaments and application to the binary cluster A 222 and A 223', Dietrich, J. P.; Schneider, P.; Clowe, D.; Romano-Díaz, E.; Kerp, J. Astronomy and Astrophysics, Volume 440, Issue 2, September III 2005, pp.453-471) had low signal-to-noise ratios or other issues.
So while mainstream media gets it wrong and declares this has never been done before, Science 2.0 writers instead get the real story.
“This is the first time [a dark matter filament] has been convincingly detected from its gravitational lensing effect," said astronomer Jörg Dietrich of the University Observatory Munich. Obviously previous researchers felt confident they had confirmed this theory of structure formation of the universe but real confirmation was believed to take much larger telescopes.
Dark-matter filaments such as the one bridging the galaxy clusters Abell 222 and Abell 223 are predicted to contain more than half of all matter in the Universe. Credit: Jörg Dietrich, University of Michigan/University Observatory Munich. Link: Nature
What did they do differently? The astronomers used observations from the Japanese Subaru telescope on Mauna Kea in Hawaii that had been unused and in an archive. As they told Space.com, they used statistical analysis of light from more than 40,000 background galaxies to determine that unseen mass in between the two galaxy clusters was warping space-time - gravitational lensing.
Citation: Jörg P. Dietrich, Norbert Werner, Douglas Clowe, Alexis Finoguenov, Tom Kitching, Lance Miller&Aurora Simionescu, 'A filament of dark matter between two clusters of galaxies', Nature (2012) doi:10.1038/nature11224