One day during his travels in the Himalayas, Vivekananda sat for meditation under a pipal tree by the side of a stream. There he experienced the oneness of the universe and man — that man is a universe in miniature. He realized that all that exists in the universe also exists in the body, and further, that the entire universe can be found contained in a single atom. He jotted down this experience in a notebook: “In the beginning was the Word, etc. The microcosm and the macrocosm are built on the same plan. Just as the individual soul is encased in the living body, so is the Universal Soul in the living Prakriti [Nature] — the objective universe… . The dual aspect of the Universal Soul is eternal. So what we perceive or feel is this combination of the Eternally Formed and the Eternally Formless.”
During Vivekananda’s itinerant days, he had various kinds of spiritual experiences. Once in a vision he saw an old man standing on the bank of the Indus chanting Vedic hymns: he distinctly heard the invocation of the Gayatri mantram from the Rig Veda. The swami believed that through this vision he had recovered the musical cadences of the early Aryans. He also experienced the presence of the Cosmic God in all beings. (Source: God Lived with Them)