Prior to STD subspace was referred to by name but not really described, but it was simple referred to as a dimension of space through which the federation would communicate. Now with Star Trek Discovery we are told the mycelium network exists in subspace. So essentially, the Federation communicates through the mycelium network. Now, in the real world scientists have discovered that fungi do communicate through their mycelium, but mycelium on earth only exists within a narrow temperature range, it really cannot exist in space, could it exist in subspace? If subspace had this narrow temperature range and spores lived in this dimension, but fungi are heterotrophs, living organisms that require nutrients from their environment and water, etc. which subspace is unlikely to have-unless this is a special type of fungi, a space fungi that lives on radiation or space debris, but then it would likely not exist everywhere throughout space, only in those parts of space that had the elements it required.