The universe consists of visible matter (the stuff we see) dark matter (matter that fills the space in our universe that we can't see at all) dark energy (which is stronger than gravity and causes the universe to expand)
42 Facts About Space, A Homage to Douglas Adams - "In physical cosmology and astronomy, dark energy is an unknown form of energy which is hypothesized to permeate all of space, tending to accelerate the expansion of the universe.
Dark Energy & Dark Matter The two largest pieces of the Universe that we know the least about, yet nothing less than the ultimate fate of the Universe will be determined by them. (Illustration: NASA/CXC/M.Weiss) The Universe in a Jelly Bean Jar
What is Dark Energy? The galaxy cluster Abell 1689 is famous for the way it bends light in a phenomenon called gravitational lensing. A new study of the cluster is revealing secrets about how dark energy shapes the universe.