I see it is actually called 'Inflation Theory'.
What I mean is that if the universe was created by the big bang 13.8 billion years ago, which is generally accepted, an if objects in space/time can travel at no more than the speed of light, then the furthest observable object can be no more than 13.8 billion years ago. The furthest observed objects fall into the 13+ billion year light travel distance. However, the red-shift of these objects suggests that they are more than twice that distance away.
Either there is something wrong with the observational techniques (which seems unlikely), or the universe itself is expanding .
The way understand it as a layman is that the universe itself is expanding but the physical mechanism that causes this to happen is posited to be dark energy. But as dark energy has yet to be observed or measured directly in any way, in my mind it is not clearly defined yet.