When I used discount online brokerages, I saw some of my more round-number trades directly listed on the level II bid-ask exchange quotes, sometimes by themselves, and sometimes grouped with other buys and sells. The more common my request and/or unround the numbers, the more likely my bid or ask got grouped with others, and the less common my request or the more round numbers, the more likely my bid or ask showed up in the on-market exchanges directly, at least for a tick or two. Sometimes my trades showed up on tape by themselves, and sometimes they showed up aggregated, and sometimes they didn't show up at all. If I entered a trade for 1,000 shares, those often showed up on tape by themselves, but not always. Fractional sub-100 share trades were most commonly aggregated in various ways, probably in-brokerage sometimes.
In essence, the online brokerages seem to do whatever they want with it, as long as they fit my order parameters. Other than that, I guess I'd have to buy a port on the NASDAQ computer routers and set up a clearing house account and any other requirements and trade directly (I haven't done that yet, so I don't know if there's any other steps, besides co-locating a computer on their floor and setting up programs to talk NASDAQ trade protocols, which means quite a bit of programming).
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Ohhhhhh ... I missed some of the context of the question. I only read two or three wikipedia-like blurbs about dark pool trading, and quite frankly, it was pretty descriptive and easy to understand: I think JBRR is right, and a quick google search would answer that for you. The Investopedia vid is pretty straightforward. None of what I described above was dark-pool. If after reading the below references, someone then experienced what I was discussing in the above two paragraphs and still thought that that was dark-pool traded, then I suggest that that person doesn't have the brain power to understand what is going on and ought to take that into consideration during their planning assessments.
Google
An Introduction To Dark Pools
Dark liquidity - Wikipedia
Dark Pool Liquidity
10 Things People Don't Get About Dark Pools
The government sent a blurb opinion; I don't know the effect:
SEC.gov | Shedding Light on Dark Pools
