Most of the universe's stuff is apparently dark matter, invisible material pervading the universe and supplying the bulk of the gravitational attraction holding galaxy clusters together. The matter contained in the visible stars in the cluster isn't enough to keep such clusters from flying apart. Now comes one of the strongest pieces of evidence for dark matter. The Hubble space telescope has found what looks like a ghostly ring, the remnant of a massive collision between two galaxy clusters sometime in the last two billion years. The ring is about 2.6 million light years across and lies about five billion light years from earth. A Johns Hopkins research team noticed the ring. They can't directly see it of course. But they infer its existence because its gravity bends the light of more distant galaxies in the background.