What a society fails to know is often not an accidental gap but something its culture produces, which can be studied as carefully as knowledge itself. Understanding what experts did not know, and why, explains as much as understanding what they did know.
Abundant, well-documented knowledge can fail to cross from one place to another even when no one bans it and the trade routes are open. Medicines to end pregnancy were widely known in the colonies and the commerce in drugs could have carried them, yet they never entered European medicine.
Knowledge stays unlearned when it belongs to a domain the people running the institutions don't occupy or value. Remedies held by midwives and female healers were passed over by male physicians and travelers who collected mainly what served their own health, in medical bodies and government offices run by men.
Powers that eagerly extracted some knowledge from colonized peoples worked just as hard to bury other knowledge that endangered their control. Medicines that ended pregnancy let enslaved women deny owners their 'property' and could let European women cut the population the state wanted, so this knowledge was pushed aside in fights over who controls women's fertility.
A supposedly neutral scientific naming system can win out not because it is truer but because those who wield it hold power, and in winning it erases where knowledge came from. A flower long used by local female healers ended up officially named after a European general, stripping away its local names and uses.