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)
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
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.
In BriefThe Dark Energy Survey recently found 11 new stellar streams in the Milky Way. Finding and understanding these streams could help us to better understand the origins of the Milky Way and the distribution of dark matter.