A concept is the relation between two or more objects or nested concepts.
Physics is concerned exclusively with objects that exist and their phenomena.
Definitions are critically important to the Scientific Method. Science requires rigorous definitions. What is a concept? As already stated, a concept is the relation between two or more objects. But there are also concepts which are the relation between concepts. There are abstract concepts, static concepts, and dynamic concepts.
Abstract concepts involve a combination of concepts and objects. Such as...
The Universe: a concept that embodies matter (atoms) and space (nothing). Or, energy: The relation between matter, its motion and the distance it has traveled.
An example of a static concept is location: the set of static distances to all other objects.
Motion is a dynamic concept: two or more locations of an object.
But there are higher order concepts or nested concepts like marriage. It is not possible for surface to surface contact between the concepts love, admiration or mutual respect which is part of a healthy marriage. There is an important distinction to be made here. The relation between concepts and concepts; the relation between objects and their phenomena. By phenomena I mean surface to surface contact between physical objects. Only physical objects can have surface to surface contact. Abstract concepts and nested concepts can not. This is why "The Universe" is not a scientific term. It can only be of use in helping to conceptualize.
A single object universe is useful only as a thought experiment. It is not expected to be taken literally. For instance, it helps one understand why it takes at least two objects for our Key Terms exist, object, concept, space and thread to make sense.
More Discussion
Whether we are considering a galaxy, a solar system, a planet, objects on the planet, men, a single man, one cell, a molecule, an element, or an atom, we are relating objects with location in respect to other objects with location. However, if we consider every object we can only relate it to space, in order to conceive of it. The unscientific term, "The Universe."