Astronomers using data from NASA's Hubble Telescope have
observed what appears to be a clump of dark matter (image above)
left behind from a wreck between massive clusters of galaxies.
The result could challenge current theories about dark matter
that predict galaxies should be anchored to the invisible
substance even during the shock of a collision. Abell 520 is a
gigantic merger of galaxy clusters located 2.4 billion light-
years away. Dark matter is not visible, although its presence
and distribution is found indirectly through its effects. Dark
matter can act like a magnifying glass, bending and distorting
light from galaxies and clusters behind it. Astronomers can use
this effect, called gravitational lensing, to infer the presence
of dark matter in massive galaxy clusters. This technique
revealed the dark matter in Abell 520 had collected into a "dark
core," containing far fewer galaxies than would be expected if
the dark matter and galaxies were anchored together. Most of the
galaxies apparently have sa