Who slapped whom?

In a train there were six passengers sitting in a coupe. A middle-aged man, a middle-aged woman, two young girls and two young men. There was hardly any conversation among them. The train passed through a tunnel. It was pitch dark for almost 90seconds. During this period somebody kissed and immediately there was a slap on somebody’s face. For some time there was complete silence but all the passengers except one were wondering about that episode. The middle-aged man and woman were thinking that one of the young men must have kissed the girl and was slapped in return. Both of the young girls were thinking that the other girl must have been kissed so she must have slapped the young man. One of the young men was wondering since he was slapped but he had not kissed the girl. Another young man was seeing the faces of all the five passengers since he had kissed on his own hand and slapped the other young man.