Often when I'm teaching an astronomy class to the general public, I begin with a “what is the universe made of?” slide. I show pretty pictures of stars, galaxies, planets, etc. and discuss all the different objects astronomers are studying in outer space. But when I do this, I'm technically lying to everyone. These observable objects actually make up only 4% of the entire universe! To be honest, astronomers have almost no idea what makes up the other 96% of our universe! The current most popular idea is that the universe is also made of 22% dark matter and 74% dark energy. Dark matter is just “stuff” distributed throughout space that interacts gravitationally with other things, but doesn't emit or interact with light. Ergo “dark” matter. Dark energy is this mysterious entity that is causing the expansion of our universe. The universe is expanding at an increasing rate, and therefore something must be “pushing” the universe, giving it energy to increase its rate of expansion. Nobody really understands what this energy is or where it comes from, but we call it “dark” energy because it must be there, we just can't see it. A bit unnerving that the people that are supposed to know everything about outer space really have no clue about 96% of it huh? So how do we “know” that this crazy dark matter and dark energy exists? Well that's a topic for upcoming ADYK posts....
image credit: Hubble/NASA
