Decent synopsis.
No one knows where the Slavs originally came from. At the time of the fall of Rome, the Germans dominated all of Europe from the Rhine to the Don River in south Russia. Beyond the Don was the domain of the ruthless Huns. What is now the Slavic part of Europe - Poland through Russia and Bulgaria - apparently did not exist. Or if it did, the Slavs were the underlings of the various German tribal confederacies - the Goths, Franks, Burgundians and Vandals, etc.
Among the theories is that the Germans displaced so heavily into the shattered Roman empire that they left parts of Europe almost completely depopulated and allowed the Slavs to take them over. (Northern Europe was sparsely populated at that time and peoples moved around far more freely.)