There is astronomical evidence that something called "dark matter" produces gravitational attraction but does not emit or absorb light. For example, observations of the motion of stars in the outer parts of galaxies suggest that each galaxy is embedded in a larger halo of dark matter. Understanding dark matter’s true nature is one of our greatest astronomical challenges. Combining our best theories and observations strongly suggest that ordinary atoms (like the oxygen, carbon and hydrogen in our bodies) comprise only about 5% of the total mass and energy in the universe. Dark matter particles are believed to make up 27% of the total mass and energy and the remaining 68% is in the form of dark energy or quintessence, discussed below.
