Compare and contrast.

From the Chicago Mercantile Exchange 2012 10-K:

Customer Base Our customer base includes professional traders, financial institutions, institutional and individual investors, major corporations, manufacturers, producers and governments.

And from the Chicago Mercantile Exchange 2013 10-K:

Customer Base Our customer base includes professional traders, financial institutions, institutional and individual investors, major corporations, manufacturers, producers, governments and central banks.

And there you have it, but in case anyone is still confused, here is some more:

At this point why even pretend there is a "market"? The "market", and by "market" we mean stocks - it has long been known that central banks actively trade bonds, FX and commodities - is whatever central banks say it is.

Finally, if central banks are going to be rigging the market as they now conclusively are on a daily basis, perhaps they can disclose ahead of the trading day start to everyone, and not just the primary dealers, what the closing S&P 500 price for any given day is.