Mathematical Physics has a handful of answers to what lies beyond the black horizon:
1. “We don’t know.”
The theorists argues that a telescope can only see so far. It is at that point that they deflect the discussion towards the Observable Universe…
2. “What exists is within the Observable Universe because what lies beyond detection doesn’t affect or concern us. It cannot be tested.”
It is irrelevant whether what lies at the other side of the Universe affects us. This doesn’t answer the instant question which is exclusively a qualitative, hypothetical issue. Does the Universe have limits?
Most mathematicians shrug and return to answer # 1.
3. “The Universe is infinite.”
The theorist insinuates that, by definition, there can be no entity or medium or object or thing encapsulating the Universe if the Universe is infinite. An infinite Universe has no borders.
Is he talking about space or about the number of atoms within space? Is space a medium like a box? Can a non-entity be infinite? Or should the word infinite be reserved strictly for objects? (… in which case, of course, there is no such thing as an infinite thing!)
Or is he saying that we would count the number of atoms incessantly? Are new atoms being created from the void? Is that why we would never stop counting them?
The Infinite Universe
Physics begins with definitions. It enables the listener to follow the speaker’s train of thought. A scientific definition is one that can be used consistently throughout the talk.
something: that which has shape (synonyms: object, thing, entity, medium, body, structure)
nothing: that which doesn’t have shape (synonyms: space, vacuum, void, nothingness)
Something and nothing are antonyms. One word means the opposite of the other. Now we can use these terms consistently (rationally, scientifically) in our discussion.
The word infinite is an adjective. As such, it should not be used to qualify actions or concepts. It is unscientific to say ‘infinite motion‘. The rational way is to say moving incessantly or perpetually. As an adjective, infinite can only be used to modify objects.
Labeling the Universe as infinite is to suggest that there is a standalone object that has no shape or borders. This contradicts the strategic definition of object. There is no standalone object that is infinite. Even if the object were expanding faster than anything can reach its borders, the object would not be infinite because it obviously has borders. What is incessant or constant is the motion of the object trying to reach the perimeter.
Universe is not a standalone object. Universe is a concept.
concept: a word that invokes or embodies two objects or two words treated as objects
Universe: matter and space
Hence, the Universe cannot be infinite even if it qualified as an object.
Beyond the black horizon
It needs no justifying that something cannot spontaneously lose length, width, and height and morph into nothing (i.e., space, vacuum). Conversely, nothing cannot spontaneously acquire shape and morph into something. The proponent of such as theory is saddled with the burden of illustrating the mechanism by which such unimaginable metamorphosis takes place.
If matter cannot convert into space and space cannot convert into matter, and if matter cannot escape from that which has no shape or borders (vacuum, space, nothing), then matter has always been there. There could not have been a moment of creation of matter because no theorist would be able to imagine, let alone explain, the physical mechanism of such surrealistic metamorphosis.
The amount of matter in existence is constant: it can neither be created nor destroyed nor made to leave space nor dissolve and become vacuum. Space, vacuum, nothingness, or whatever we care to call it, has no borders and is not made of anything. Space is not a physical object. The devil’s advocate merely has to draw a picture of space and identify the medium or entity that encapsulates and gives shape to space.
And as for why all the matter in existence, it has always been there. Matter had no beginning and will have no end. In this sense, the Universe is definitely eternal (as opposed to infinite). Father Universe was never born and will never die.
We do not have to ‘test’ any of this with an experiment, for instance, by traveling beyond the observable universe. This is a conceptual issue that rational individuals can reason out on their own down here on Earth.