You idiot, do you even know anything about Islam in western Africa? Have you ever met a west African Muslim before? There was no Arab conquerors involved in converting the west Africans to Islam. There was never any Arabic colonies also. The reason that Islam spread in West Africa is that African kings and the rulings class accepted Islam by being in constant contact with Muslim merchant-scholars in turn the masses in the region accepted Islam.
1) Muslim merchant-scholars from north Africa assisted the African Kings in trade and commerce (this is nothing new, this happened all over the world from China to Indonesia)
2) Many of these merchants were also spreading the word of Islam, The African Kings either for economical, political or spiritual reasons accepted Islam
3) The ruling class of the African society accepted Islam, then after this the rest of society.
This is the same reason that Indonesia is the largest Muslim country in the world, not because of Arab conquest. If you look at all of the west African nations that practice Islam, there is no influence of Arabic in there language. Arabs controlled Spain for hundreds of years, but you can see the Arabic influence in there language and culture. Do some research and get your facts right.
Islam in Gambia | History
FSI | SPICE - The Spread of Islam in West Africa: Containment, Mixing, and Reform from the Eighth to the Twentieth Century