One can only speculate, but to me it appears to be lubricant that has evaporated and redeposited on the back of the lens surface. Maybe the camera got really hot at some point?
Does not appear to be fungus, but again, it is pure speculation on my part.
The only sure way to tell is to have the element removed and attempt to clean it IF the spotting is not on a cemented surface in between elements.
Have you shot anything with it? It may not matter much in any event unless you are shooting directly into sunlight with your subject back lighted.