Bad sites can't sell any inventory if they honestly declared the domain name. So they lie. They pretend to be mainstream publishers in order to sell inventory on exchanges. Good publishers are harmed when ad dollars chase this "cheaper inventory" on exchanges (because the ad dollars are siphoned away to these fraudsters). Advertisers and marketers are harmed because they thought they bought from a legit site, but instead their ads were wasted on bots and fake sites.

1. April 2017marketing.scienceconsulting group, inc.
linkedin.com/in/augustinefou
“Dark Revenue” from Fake Inventory
“Dark revenue” is the ad revenue diverted away from
legitimate publishers by fake sites pretending to be them.
A fake site or app will never be able to sell their ad impressions in exchanges if they
used their actual domain or app name, like the examples below. So they pass a
false variable into the bid request and pretend to be a legitimate site like ESPN.com
that is in high demand. Advertisers then show their ads, but they may never find
out that their ads were not actually shown on the legit site, but on the fake site. If
the buyer had fraud detection ride along with the ad, they may see high bots and
fraud (because the ad was shown on fake site with no human audiences). Another
problem with this is when buyers call up the real publishers accusing them of fraud.
Fake Sites Source: Sadbottrue.com Fake Apps
How big of a problem is “dark revenue?” Very big, but it depends on the size of the
legitimate publisher. We looked up the inventory available on open exchanges for
small, medium, and large publishers (30 day period), where we knew the actual
numbers of pageviews and impressions. The multiples below is how much larger
“dark revenue” was compared with the actual impressions sold by the publisher.
KEY FINDINGS
• Large pubs (>500M /mo) – “dark” is 1-2X
• Medium pubs (10 – 500M /mo) – “dark” is 5-10X
• Small pubs (<10M /mo) – “dark” is 20-100X
In some cases, those publishers didn’t sell ANY inventory on exchanges, so ALL of
that inventory on exchanges was fake.