As scientists have learned more about the universe, they have come to the conclusion that there is more matter in the universe that what scientists can see. Most objects in the universe emit or reflect light, but there is a large amount of matter in the universe that doesn't. Scientists aren't exactly sure what makes up most dark matter. Dark matter may be normal matter, like brown dwarfs, or could be something we've never encounter before.

Scientists believe that not only is the universe expanding, but the rate that the universe is expanding is increasing. The only way to explain this phenomenon is that there is a large amount of energy unaccounted for. Scientists call whatever is causing the universe to expand faster dark energy.