The ancient Proto-Indo-Europeans

I wonder who the ancient Proto-Indo-Europeans were, why they migrated across such vast continents, their influence on the cultures and languages, and how many they were.

Opinions seem to range widely on this subject, from that the Proto-Indo-Europeans were some master people that exterminated the indigenous peoples, to that they did not exist at all, and that all the various cultures and languages under "Indo-European" are indigenous to their regions.

I understand that the Proto-Indo-Europeans were not a single tribe, but rather a group of tribes who shared the same language, culture and religion, much like the various Greek city states did (they were rarely united). It also appears that the Kurgan hypothesis is more supported by the evidence than the Anatolian hypothesis.

How did the Indo-Europeanization take place? Was it only the culture that spread, or was it that Proto-Indo-Europeans conquered and subjugated indigenous peoples, and that hybrid cultures were thus formed, much in the same way that Hellenistic culture formed (it was a mix of Greek and Near Eastern cultures)?

How many of the modern Europeans descend from Proto-Indo-Europeans? At least according to what I've read, 80% of Europeans have Paleolithic roots in Europe, and surely not all of the remaining 20% can be from the Proto-Indo-Europeans. So with this numbers, the subjugation theory seems unlikely, and many people would have had to be supressed. But that it was only a cultural spread doesn't make much sense either, because of the huge areas that became Indo-Europeanized, and that so many aspects of Indo-European culture was adopted.

Wikipedia has some nice entries on this, but does anyone here know more?

And I can't avoid to think of how much the Proto-Indo-Europeans and their culture have affected human history. They would easily qualify as the most influental people in history.

It's a possibility that they were the first humanity to create the first civilization around the dried Mediteranean Sea and Black Sea... known by some as Atlantis... before the Ice Age ended and flooding the Mediteranean Sea...