This is probably a question for /r/Linguistics but Proto-Indo-European ("PIE") was one of the first accepted proto languages by linguists during the 18th & 19th Centuries. The "Kurgan hypothesis" attempts to describe the origins of PIE & the ways in which it spread.

It postulates that PIE originated within the Kurgan culture of the "Pontic-Caspian Steppe" which geographically stretches from Eastern Romania to the Ural mountains. That's all I can say on the subject but I would recommend reposting in /r/Linguistics.

I also recommend reading The Kurgan Culture and the Indo-Europeanization of Europe by Miriam Robbins-Dexter