Astronomers see the invisible.
By observing a rare collision of galaxies, Marusa Bradac of the
Kavli Institute for Particle Astrophysics and Cosmology at Stanford,
along with colleagues, have made the first direct detection of
"dark matter" by exploiting one of its few visible effects:
gravitational lensing. When the clusters collided, the volumes of gas
were slowed by the impact, but the associated dark matter from each
cluster presumably continued to speed along unimpeded. Comparisons of
images from NASA's Chandra X-ray Observatory, the Hubble Space
Telescope and other instruments, revealed that the clumps of hot,
shocked gas from the collision were in a separate part of the sky from
the greatest lensing, which was caused by the unseen dark matter.