Hi Fami! The contraction It's (It is) alludes to the pronoun It (the dark). The use of "It's what we know" in that example means that we are comfortable with "it" in a familiar way. We could say something similar such as, "I'm not afraid of the dark, I'm used to it, it's like an old friend."

in english, there are a lot of phrases that can be interpreted multiple ways. in this instance “it’s what we know”, could refer to both only knowing the dark and only liking the dark. i think the author means “knowing the dark”, they like being dark because they have yet to experience light. this could be thought of both literal and metaphorical (literal being that they’re used to being in dark, dim light areas. metaphorical meaning they have yet to see any good days or experience profound happiness.) let me know if i helped u!! ❤️