Another chunk of matter has been found, via the absorption of x-rays by Oxygen VII ions. Basically, it is intergalactic ionized gas. From the link:
Now David Buote, an astrophysicist at the University of California, Irvine, and an international team report the detection of x-rays absorbed by such missing ordinary matter. Using NASA’s Chandra telescope and the European Space Agency’s XMM-Newton, the team spotted this gas along a portion of the Sculptor Wall, part of a large assemblage of galaxies some 400 million light-years away. Oxygen VII between the galaxies absorbed x-rays coming from an energetic galaxy behind the Sculptor Wall. Buote gives their detection a 99.7% chance of being correct.
The team’s observations, which will be published in the 20 April issue of the Astrophysical Journal, appear to be “well-planned, careful, and pretty convincing,” says Michael Vogeley, an astronomer at Drexel University in Philadelphia, Pennsylvania, who was not involved in the study. “The finding that much of the regular stuff is WHIM-sical and that it is associated with the largest-scale structures seen in galaxies is a confirmation of the basic picture of how the regular stuff in the universe is predicted to behave in such a universe.”
Still not enough to negate the need for dark matter, but a good observational result.