There is a movement in the UK nowadays to expose the slave-owning history of prominent people and businesses and institutions that sent slaves to the Americas. What is not acknowledged is the similar or greater slave-owning history that sent slaves not west, but east.
Slavery from Sub-Saharan Africa goes back millennia. Until the start of the Arab/Islamic slave trade in the 7th century of the Common Era it was localised and slaves were typically captives in war, criminals, or those in debt.
As Islam expanded in the 7th century, Arab traders began trafficking Africans north across the Sahara, and eastwards.
Slaves transported eastward were brought from the interior of Africa (especially Tanzania, Malawi, Mozambique, Congo), and trafficked over the Indian Ocean to the Middle East, India, Persia, Indonesia, and China, and in large numbers.
The transatlantic Slave Trade ran for a much shorter period, from the 15th to the 19th century. The Portuguese began it, and it was later dominated by British, French, Dutch, and Spanish traders. Over 12 million Africans were captured through raids, wars, or sold by other Africans and forcibly taken across the Atlantic Ocean to the Americas from major slave-trading ports in Ghana, Benin, Nigeria, and Angola.
The estimates of slaves transported sent eastwards ranges from 10–18 million people.
The wide range reflects that fact that many of the Arab-led slave trades had no documentation, unlike the Transatlantic slave trade run by Europeans who kept detailed shipping records.
The wide range of estimates of the eastern trade also reflects the centuries-long time scale over which the trade occurred.
And a third reason is the lack of will to describe it, unlike the analyses in the West, beginning after the abolition of slavery.
All in all though, and this is a surprise to some, the eastward slave trade was the equal to and likely more in numbers of slaves than the Transatlantic trade.