With shadow biosphere is defined an hypothetical microbial biosphere of Earth that uses biochemical and molecular processes radically different than currently known life. Even if life on Earth is relatively well studied, the shadow biosphere can still be remained unknown, due to selective exploration of the microbial world objectives, firstly, the biochemistry of macro-organisms.
On Earth so, in theory we could co-inhabit without our knowledge with microbial life-forms with a biochemistry completely different from those shared by life as we know it today.