Perhaps it started when the first human looked to the skies and saw shapes of impossible beasts in the clouds. Maybe it was earlier, when nightly visions fired Mankind's new-found imagination to look beyond the established way of doing things, to seek out the new and different. Maybe, in doing so, our species just discovered what other races on distant worlds have known of for eons. The dreamworld, insubstantial and shifting though it is, has been with us since before time. When we have gone, it may survive still, in the thoughts and hopes of species undiscovered.
Dreams are infinite, so long as there are dreamers.
However, our contact with it is threatened. Though the world of dreams is infinite, our own contact tends to be fleeting. Children are often touched by the dreamworld, but, as we grow up, many succumb to pressure to abandon this place in favor of Reality. Many people believe, falsely, that imagination is a distraction from More Important Things, failing to realize that, without it, we wouldn't be where we are now as a society. The dreams and visions of those who saw beyond the here and now of Reality.