Astronomers have learned that there's a lot more to the universe than what we can see directly. It's like an iceberg. The part of the iceberg that's visible floating above the surface is only one-tenth of all the ice. The other nine-tenths lurks underwater, unseen. In our universe, the gas, stars, and galaxies we can see make up about 4 percent of what is out there. We know there's more we can't see because the unseen "stuff" exerts a gravitational force. It pulls on the stuff we can see. But gravity provides our only clue. The unseen stuff doesn't emit radiation that we can detect, so astronomers call it dark matter. What is dark matter made of? We don't know. It's not just dark stars or planets or even black holes. It may be vast numbers of tiny particles. We do know that, whatever it is, it makes up about 23 percent of the universe. So if regular matter and dark matter together are about 27 percent of the universe, what makes up the other 73 percent? An even more mysterious thing called dark energy. Until the late 1990s, we didn't even know it existed. Then astronomers discovered, to their amazement, that the universe was not only expanding, but speeding up as it expanded. We still don't know the source of the energy that is powering the speed. Is it some new kind of energy field, or a property of space itself? Or are we completely mistaken about some basic facts of physics and gravity? Will dark energy make physicists rewrite the laws of physics in order to understand the universe? One thing we do know: the universe is much stranger than we ever imagined.