Dark pools exist because these "real investors" you speak of are muppets. If you're a market maker out in the big bad world you are exposed to adverse selection risk - i.e. you're up against smarter, better informed or faster traders who hurt your profits. If you set up a dark pool, you can pick and choose your customers, and keep these people out. You can quote tighter in the dark pool than the wider market, so your customers think they're getting a great deal, but in reality they're only getting this deal because they are (collectively) great big juicy marks who you can make a fortune from.
This is why you see the big investment banks setting up dark pools, they can't compete in the technology race or with smaller more agile firms, so they retreat to these pools.
This is why you see the big investment banks setting up dark pools, they can't compete in the technology race or with smaller more agile firms, so they retreat to these pools.