There are several realms which ordinary persons do not perceive. Because they cannot see them, that doesn't mean they don't exist. One of these realms is the sambhogakaya that can only be visited by highly realised Bodhisattvas. In the pure realm of the sambhogakaya, the Dharma is continuously taught. One sambhogakaya realm is Tushita, which is presided over by the next Buddha, the Maitreya Buddha. Buddha Shakyamuni dwelled there before coming to earth to give Dharma teachings.

-- Thrangu Rinpoche