Consciousness can be conceptualized as an omnipresent feild that is the internal witness in all forms.
It is always one with energy, so its never seperate from Nature, like sweetness in a candy.
The subtle aspect of nature as a whole.
Millions of people have experiential evidence, testamony. With reason these people have found words to describe the non dual experience of awareness. You see that you are not "this body" but your "all bodies".
It used to be based solely on faith in experience, but nowadays several conclusions have been theorized based on mathmatics, and astrophysics. Namely Quantum physics, and electromagnetic feild theory.
Im not really qualified to discuss these scientiic aspects in detail, sorry.
But the reason for belief in this theory is experience plus reasonable theories(such as QPh).