Einstein already proved that matter and energy are interchangeable, which is something that is now scientifically accepted; despite it being obvious from the fact that we eat matter and our bodies convert it into energy in order to move.
But what about time and space, and how do they relate to matter and energy?
In a simpler universe it becomes more obvious. If I have three hypothetical objects in a universe at x distance apart then time and space become intrinsically related. In this scenario there are two objects stationary to one another, relatively speaking, and a third object which moves from the position of the first object to the position of the second object. A starting point (point A), an ending point (point B) and a moving object (point C).
If I double the distance between A and B then that is identical to halving the energy in the universe. If I slow down time in the universe then that is identical to increasing the distance between A and B, or alternatively: halving the energy again. There is literally no difference between increasing distance or decreasing the speed of (or allowed) time, no difference is made to the universe or any calculations made therein, subjective or objective.
That makes Matter = Energy; Energy = Space; Space = Time. Therefore matter, energy, space and time are all interchangeable characteristics, which implies strongly that they are all forms of one thing.
How about if the three objects in the universe were all moving relative to one another? If every object in the universe is moving at the same speed in the same direction then, since there is no point of reference, this universe is entirely static. This universe, in which there is no relative movement, is exactly the same as universe in which there is no time, or a universe in which there is no energy. That makes time an aspect of distance and energy.
Furthermore, in such a static universe how could we really say there are any objects? Without any movement or without any time then any interaction between objects becomes impossible. Without interaction no object would ever know there are other objects, and no object would ever know even its own existence. Considering that any interaction is impossible then to have no time or no movement is the same as a universe in which there is no matter, no energy, no time, or no space. All of these universes are identical.
Matter, energy, space and time can only exist therefore, if there is movement. A universe without movement but with time is identical to a universe without time but with movement, or a universe with no matter, or a universe with no energy, or a universe with no space. As soon as one object begins to move relative to another object then time suddenly comes into play, matter suddenly appears to exist, and space is now measurable as the distance between two objects as measured by a third observer object.
The common denominator here is movement, specifically relative movement. Even energy itself only exists as an aspect of relative movement, movement being all energy is. So we could say the true nature of existence is relative movement, which is a subjective relationship between two points of reference.
Perhaps then we could say that the only thing that exists is subjective relationships, and everything else is an illusion.