Dark matter, as originally hypothesized, is extra hidden mass that astrophysicists calculate is necessary for holding together fast-turning galaxies. The most popular notion is that this matter is made of some yet-to-be-identified particle that has almost no interactions with light or ordinary matter. Yet it seems to be everywhere, acting as a scaffolding for galaxy clusters and the whole structure of the universe.

On the other hand, dark energy is needed to explain the more recently-discovered acceleration of the universe’s expansion. It supposedly exists all throughout space, delivering a pressure that counteracts gravity. (via SPACE.com)