Time is actually a dimension, a direction that things in the universe can travel in and occupy. Just as objects in the universe can move up and down, forward and backward, or side to side, objects can also move through time. Unlike the three spatial dimensions, however, different kinds of objects in our universe move through time in only specific directions. Mathematically, it is correct to say that matter—galaxies, stars, planets, and people—only move forward in time. Meanwhile, particles made of antimatter only move backward in time; and particles of energy—such as photons, which have no mass—do not move in time.