The Fifth Dimension is The Twilight Zone.

As defined by The Narrator:

"There is a fifth dimension beyond that which is known to man. It is a dimension as vast as space and timeless as infinity. It is the middle ground between light and shadow, between science and superstition, and it lies between the pit of man's fears and the summit of his knowledge. This is the dimension of imagination. It is an area we call the Twilight Zone."

Awareness of this fifth dimension did not occur until 1959, when it's existence was first introduced by the Narrator. (TZ1: "Where is Everybody?")