Complex sentence: It is assumed that the chemical space exceeds 10 60 compounds and it seems quite unlikely that the mankind will be ever able to create all those molecules by knowing that only 27,000,000 molecules have been registered (and been made) so far.
Split into simpler sentences: It is assumed that the chemical space exceeds 10 60 compounds and mankind will therefore never be able to create all those molecules. This becomes evident when knowing that only 27,000,000 molecules have been registered (and been made) so far.