Galactic smashup leaves dark matter debris

Amid the wreckage of a massive galactic collision lies an enigmatic heart of darkness —a clump of dark matter. Normally, dark matter — the unknown substance that makes up the majority of the universe’s mass — remains anchored to galaxies. But in Abell 520, a galactic cluster sometimes known as the “train wreck,” dark matter appears to have collected in the core even though the galaxies had moved on after colliding.