When did yogurt become popular?

The development of yogurt happened in prehistoric times. The bacteria that thicken milk in yogurt are usually found in nature on the nipples of dairy animals, so the thickening of milk usually (or at least often) happens spontaneously when milk is stored for a short period of time.

It was first mentioned by Pliny the Elder (23-79 CE), according to Wikipedia, though there is no source cited there. His reference says that certain tribal groups made it, though it does not say it was unknown in his own culture.