At the risk of stirring controversy, "we" (and here I am assuming you were educated in a Western country, and are likely white, not that there's anything wrong with that nor is any of this your fault, and if I'm wrong I apologize for assuming but it doesn't change any of the below) never hear about African empires because historical education in the West is highly ethnocentric, basing curricula on the Middle East and Europe, the so-called "twin pillars" of Western civilization (i.e., the Biblical and Graeco-Roman traditions). I'm not sure why certain East Asian empires have a greater hold on the public Western consciousness than African empires, but I would venture that relative global economic prominence of countries like China and Japan have led to more exposure to the culture (and pop culture) of these regions in the Western imagination in the last, oh, 60-70 years.

I suspect you are already familiar with a number of African empires. Egypt, remember, is on the continent of Africa, and almost everyone has heard of the ancient Egyptian empire (more accurately, some or all of the ancient Egyptian empires). The Carthaginian empire was also African - Carthage was located in modern day Tunisia, in North Africa, although their language originated from the Middle East.

Other African empires include Aksum, Mali, and the Ethiopian Empire, all of which reached heights comparable to many well-known European and Asian empires, if not the biggest in history, like the Mongolian or Roman empires (which, of course, no empire could reach, by definition, or they wouldn't have been the biggest).

I'm no expert on ancient African history, just an enthusiast, but I thought I would point out a couple and make a suggestion as to why many people know less about African history than that of other regions.