Epicurus has no direct observational evidence of atoms or void, but he found strong circumstantial evidence from the observation of other things which he could observe directly. We therefore logically do not require direct evidence on all issues, but we require that any theories be consistent with what evidence does exist.
Just an aside here -- of what might have caused the theory of atoms -- "visual snow" --
"Visual snow (VS) is a form of visual hallucination that is characterized by the perception of small, bilateral, simultaneous, diffuse, mobile, asynchronous dots usually throughout the entire visual field, but it can be partial, and it is present in all conditions of illumination, even with the eyes closed." -- which is listed as an eye disorder.
However -- it could come on during states of meditation.