1) A black hole forms when a large star runs out of fuel and collapses in on itself, leaving an intensely dense object with strong gravity from which not even light can escape. 2) Telescopes detect radiation and x-rays from black holes, which reveal their location and size. It is believed that supermassive black holes exist at the center of most galaxies. 3) Dark matter makes up 30% of the universe but is invisible and its true nature remains unknown, though it is thought to interact through gravity. Dark energy is the dominant force driving the expansion of the universe.