How do we make sense of change in the world? We need to dream up a theory that explains what we see. We can understand why the black dots move in the way they do if we hypothesize that the dots reside on the apexes of invisible moving equilateral triangles. We might not be able to perceive the triangles, perhaps not even in principle, but we can confidently infer their existence.
But wait!
The movement of the black dots is equally explicable if we imagine the dots reside at the apexes of invisible moving squares. Or, if they are moving along the lines constituting a star shape. If we are unable, perhaps even in principle, of perceiving any of these shapes, then how could we know which theory is correct? Indeed, perhaps none of them are?
Likewise, change in our world is accounted for by the existence and interactions of subatomic particles. We cannot see these particles, not even in principle, but we can infer their existence, just as we can infer triangles for the movement of the black dots. So, does this mean that what we see with our naked eyes could appeal to quite different entities to explain what we see? I think so.
This is a problem in science since all possible evidence we could have radically underdetermines which theory is the correct one. This is called the underdetermination of scientific theory by evidence. It also invites the question of whether our theories that utilize entities that can never be directly observed, actually depict a literal state of affairs.