It has been reported that a new antibiotic was ‘discovered using AI’. This needs a bit of unpacking. Finding any new drug means searching through ‘chemical space’ – the many possible configurations of atoms that can make up molecules. It’s difficult to get a grip on how vast this universe of possibility is. Most drugs consist of molecules with fewer than thirty atoms and a molecular mass of less than 500 daltons (a hydrogen atom has a mass of one dalton, give or take). It’s hard to estimate, but even if you restrict yourself to a handful of elements (carbon, hydrogen, oxygen, nitrogen, sulphur) there are at least 1060 possible molecules that fit these criteria. This is a big number, more than a thousand times the number of hydrogen atoms in the Sun. Exploring this chemical universe in its entirety is impossible. The hope is that using predictive algorithms from machine learning can help guide you to the right galaxy.