Was there really an IE invasion into Europe? If so, where are the mass graves and ruined towns that always accompany such events? Why has the genetic makeup of Europeans remained relatively stable since at least the Neolithic period, probably earlier if there was the sort of population replacement that would have been necessary for a linguistic replacement? IMHO, an IE in Europe (possibly Lithuania) fits the evidence better.
A very astute question. There is very good evidence of "waves" of peoples moving westward into Europe. Among the first of these tribal peoples were the Paleolithic peoples these folk were then marginalized or absorbed into the Mesolithic groups that followed. These groups seem to have evolved into the Neolithic until they to were narginalized by the early Beaker peoples who migrated from the northeast in to Britain feeling pressure from the east. The beaker peoples were marginalized to the far reaches of Britain, Ireland and smaller islands. The Iron Age peoples that followed them west ward were also being pushed from the east. The Gaulic and Franks, angles and Saxons ranged through Europe and they too were pushed to the margins by Alexander the Great and his father before him. The Greeks, Romans pushed the people to the edges of Europe as well. If one looks at the dress, instruments, language, ans social structures of the Gallic peoples of Cornwall, Wales, Scotland and Ireland, they bear a remarkable similarity to other indo-European groups. The kilt, bagpipes, tam, style of shoe construction, etc... The Greeks, Persians, and small enclaves in India all have similar commonalities.
These invasions were far from bloodless but these groups were small in number and reasonably peaceful. Simply looking for a safe place to live. Over time they commingled, formed bonds across cultures which has lead to a European genetic phenotype. Some smaller groups like the Gypsies, Jews and Norse genetically stand out for cultural reasons. They tended not to marry out.
No mass graves. Small groups do not lend themselves to mass graves, and living in timber homes or wattle and daub homes which can burn down on there own accord makes it hard to determine warfare or natural occurence. But if you want mass graves you can find them as a result of the Black Death.