Knowledge Of Reality
Scientifically, the universe is thought of as four dimensional (there may be more, but let's set that aside). Space and time. Both were created at the start of the universe, so the start doesn't have a time and place, as such, as space and time were created then (it's important to note that words like 'created' and 'start' have inherent temporal qualities that make them not quite correct when talking about the 'creation of time'! It is perhaps more useful to think of the universe as a sculpture and the start as an edge).
Einstein showed that object that move under the influence of gravity were objects moving in a straight line along curved space-time. This instantly links space and time as one entity. Just as space can be distant, so can time. We can't instantly see distant space, and we can't instantly see distant time but we can see both, eventually, within certain defined limits. Crucially, the contents of space, whether detectable or beyond our detection, must exist in analogue with time. Thus if distant things exist in space, then distant things must exist in time, so, in a space-time universe, the future must pre-exist.
Some things in the universe we can see and measure. Some things are beyond our detection, they might be beyond our vision, as it takes time for light to travel to us. Einstein also showed that the speed of light was also a speed of information, and that information can't move faster that light. Thus we can't see what a distant star is exactly like now, just how it was when its light shone towards us. We can predict how it should look now, with reference to other stars, to knowledge. We can imagine what it looks like now, but we can never see definitively what it looks like now.
Things beyond this limit are unknowable. Do these unknowable things exist? These things exist when we know them, they become real when we know about them. If we close our eyes, in some ways the visible universe vanishes, but it snaps into reality again when we open them. If every eye in the universe closed, then would light exist? Not as a visual medium, but perhaps these blind people would feel the warmth of light, or invent radios or machines to detect the light. If every eye in the universe closed, and every machine and feeling capable of detecting light was switched off, would light exist? No. But when an eye was opened and saw light, then it would. In another example, there are no dragons in the world. The moment one is perceived, there would be one. The existence of dragons depends on a detection and validation of them. This is the definition of what is real.
Just imagining dragons doesn't make them real, just as calculating what a distant star should look like now doesn't make that reality. These are abstract thoughts, predictions, possibilities, not actual perceptions. What if a hallucinating person thought he saw a dragon? For that person, the dragon would exist, but upon verification by others, it would not. Reality hangs on these thin threads.