所知 follows the 所-construction: 所 + verb turns the verb into a noun phrase ('that which is X-ed'). So 所知 = 'that which is known'. The pattern 据(某人)所知 ('as far as X knows') is the most common modern use.
usage
Frequently used in fixed phrases: 据我所知 (as far as I know), 众所周知 (zhòngsuǒzhōuzhī, as everyone knows), 一无所知 (yīwúsuǒzhī, to know nothing at all).
