The concept of atman, this important precept of spirituality cannot be understood with the lens of science. Science has its limitations. Science demands proofs at each and every stage of life. Proofs which are not forthcoming in the field of pure spirituality!
In the science of spirituality everything is directly governed by absolute faith in god (called shraddha in Hinduism). In absence of faith in God and his system (world of atmans) nothing much could be realized on path of pure spirituality.
If we desire reaching God in our present life time, 100% faith in God is an absolute must. We as human beings exist because God, Brahman the creator of entire cosmos exists.
Seekers, travelers of spiritual path do not know anything about god yet they pursued god, indulged in spiritual routines of life, unmindful of the fact that spiritual journey is a journey into the unknown. Believing in God and his world of atmans, seekers covered spiritual journey towards their inner self existing in heart.
It can rightly be said spirituality starts from the point where science ends. The most science could do was reach God particle. Science with its limitations would never reach world of atmans.
Similar as one can detect presence of a flower only by its fragrance, spiritualists, seekers detected presence of God in day-to-day affairs of life. The presence of atman in our heart can be realized only by demolition of ego in totality, never before. Furthermore, we should have the capability to indulge in truth 100%.
When six years of age I had zero ego! I also possessed 100% faith in God. I also practiced truthfulness 100%. From six years of age I could distinctly talk to atman existing in my heart. Allegorically our atman existing in heart is Lord Krishna giving his dictates from within.