What exactly is the fluid within a zit made up of?

Sebum. Sebum is the fatty yellow secretion of (sebaceous) glands in our skin. Some areas of the body have more sebum-producing pores, like the face, so when puberty stimulates the glands to increase sebum, and if skin pores have not yet grown to "catch up" with the increased sebum, or if there is inflammation that blocks the pores, a zit results. Sebum also contains skin bacteria so infection can occur.