Dark matter is basically stuff that influences gravity, but doesn't give off electromagnetic radiation (light). We can't observe it, however a substance like dark matter is necessary to explain certain phenomena that occurs in the universe. The easiest way we can estimate the amount of dark matter is by looking at the rotation of galaxies. If we think of a tether ball, when the ball is closer to the center it moves faster than when it is farther out. Similarly, a galaxy's rotation should decrease as one looks farther away from the center, however, it doesn't, so we can explain this inconsistency through matter that influences gravity, that we can't see.