If you were to google (search) for index funds, you will find a great number of answers. You will find some great companies offering what wasn't available just 40 years ago, an index fund.
If you search wikipedia for index fund, you will get a great explanation and a wonderful history lesson. It was only 1975 when John Bogle 'invented' and marketed the index fund to the public. Most thought he was crazy, why would investors be happy with 'average' returns. As it turns out, Bogle was crazy like a fox and now, some 35 years, investors are finally wising up to the fact that index funds routinely beat most mutual funds and certainly all stockbrokers and financial advisors.
The reason for the popularity of index funds is simple; ease of use and better performance. Add to that there is no conflict of interest as there can be utilizing a stockbroker or that you dont get caught up wasting time trying to chase the next best mutual fund, its easy to see.