The most accepted ethnogenesis theory is the Przeworsk-Zarubinsky culture, centered around what is now abandoned Pripyat (Northern Ukraine, Southern Belarus). Slavs lived mainly in swamps and heavily forested areas where they were safe from Nomadic invaders including Huns.

There was a theory that said it was because of their harsh conditions that the Slavs decided to create a quite democratic society - if they wanted to survive, they had to make sure centralization wouldn't occur, as that would create an already difficult existence even more difficult, creating a system that would make sure everyone partakes equally. Now back to the theory.

Slavs lived in that area for centuries dating back to appx. 1000 BC, where they procreated and later on had a population explosion, expanding to the south and creating the majority of peasants in the kingdom of Vandals (there are actual historical sources for this - try searching for Making of the Slavs.pdf), to the north where the heavily forested areas and swamps were just like back home but somewhat colder. There they faced Finnic, Baltic and Uralic tribes, effectively overcoming all three sorts, one of them being Chuds.

Chuds are a Finnic tribe that still exists but is on the verge of extinction, they have some folk ballads about how Chud women would hide from Slavs with their children and they drowned themselves and the children so they wouldn't take her. Meanwhile Slavs likened Chuds to "beautiful" (Chudesny - Чудесный - Beautiful, but also Strange - which might be more likely the scenario as Nemec [Mute] was used for any foreigner they couldn't understand so they might've used that to describe the Finnic people), so I guess they really liked their women(?) and the Old Rus folk legends refer to that.

At that point in time there were countless Slavic tribes that expanded to the West, the East, the North and then to the South. The explosion in their population demanded they find new and better lands, where they procreated even more (and assimilated - Slavs liked to assimilate other cultures, or less likely exterminated the local populace) but were at the height of their extent in the Dark Ages, when the Frankish Kingdom and Avars started their dominion over West Slavs (incl. what are now South Slavs).