From: mpc755 on
'Hubble Finds Ghostly Ring of Dark Matter'
http://www.nasa.gov/mission_pages/hubble/news/dark_matter_ring_feature.html

"Astronomers using NASA’s Hubble Space Telescope got a first-hand view
of how dark matter behaves during a titanic collision between two
galaxy clusters. The wreck created a ripple of dark matter, which is
somewhat similar to a ripple formed in a pond when a rock hits the
water."

The ripple will eventually reach the Earth and this is evidence dark
matter exists from the galaxy cluster to the Earth. This is evidence
dark matter is the medium of space in which light waves propagate.