They can also help trace the amounts of invisible dark matter in the clusters, possibly giving a clue to how this mysterious substance behaves. (newscientist.com)
Only a small percentage is visible as the stars, gas and dust; the rest is held in a large yet invisible dark matter halo. (science.howstuffworks.com)
This force would act only on invisible dark matter, the enigmatic stuff that makes up 86 per cent of the mass in the universe. (newscientist.com)