Dark matter and dark energy are the yin and yang of the cosmos. Dark matter produces an attractive force (gravity), while dark energy produces a repulsive force (antigravity). Together, they make up 96 percent of the universe—and we can’t see either. Astronomers know dark matter exists because visible matter doesn’t have enough gravitational muster to hold galaxies together. What’s it made of? There are two adorably acronymed leading candidates: super-dense astronomical bodies called massive astrophysical compact halo objects (MACHOs) and weakly interacting massive particles (WIMPs). Dark energy, on the other hand, is why our universe is expanding. In fact, in 1998, astronomers studying distant supernovae were shocked to learn that, around 7.5 billion years after the Big Bang, the universe began expanding faster. That indicates some unknown force is fighting gravity’s pull, causing galaxies to speed apart from one another. Think of the universe as an elastic gym band and dark energy as the Incredible (and unseen) Hulk who keeps tugging at it.