Not exactly. It took them a while to figure out what the phenomenon was and to repeat it on purpose.
This was 'observed' by it's measurable effects on something or some radiation.
That doesn't make it 'visible'.
Invisible means something specific.
What makes you think it's nature is unknown? It is known to stop certain particles.
/johnny
Its believed to stop certain particles.
The true course of the particles may simply differ from what was believed.