The idea of indexing doesn't require a specific index. It is more general than that. The idea that average returns are pretty good, and average returns with low cost-of-management are even better.

In order to get average returns, the key is simply to own a tiny fraction of the entire market, whatever "market" means to you. The focus has long been on the S&P simply because it has historically done a pretty good job of representing the US equities market, it is market-cap weighted (unlike the Dow), and the earliest index funds tracked the S&P.

It's my expectation, stated without proof, that it should be easy to find a comparable index in Europe, if you want to match the average European return.