Dark UX is the use of UX cues to nudge the user to take actions within your site or app that the company wants them to take, but which they don't necessarily want to take themselves. Let's start off with one of the most blatant examples.
You are looking for an answer on Google, and based on the URL and preview text, select a page where you think you'll find the answer. You get through the article's introduction but on your first scroll a gigantic email subscription prompt pops up. You are just trying to find a quick answer; you don't want this random website emailing you three times a day! And then you notice your two options: "YES! I want to become the next tech genius," and "No, I want to be a stupid dumb-dumb for the rest of my life and fail at all my endeavors." This blatant attempt at trying to manipulate an answer is really off-putting. You want to smash the "no" option even harder.
But what about more subtle ways websites try to manipulate our behavior? Ever notice that Netflix's "Are you still watching this?" prompt comes up after the hook of a brand new episode and not at a natural stopping point? Another type of forced continuity is when a free trial requires you to input your credit card so they can automatically charge you when your trial ends.
Or have you ever noticed how easy it is to sign up for Instagram, but how difficult it is to shut down your account? You can't even do it within the Instagram app; you have to do it through Facebook's management tools. And those settings are buried deep!
Have you noticed that for certain items on Amazon, the option for a recurring subscription of that item is checked automatically instead of a one-time purchase? The website makes it so you have to pay attention when you add it to your cart, and so you have to take extra steps not to be stuck in recurring charges.
These are just a handful of examples of the different ways that websites try to manipulate users' behaviors in ways that are to the benefit of the company and not so much to the benefit of the customer. Yeah, it feels shady, but what's the actual outcome? A lot of times it does lead to more conversions. On one level, dark UX works. That is to say, it often successfully tricks people. But it also leaves your customers with a bad impression and broken trust. Trust is very hard to win, especially in a world where there are a thousand other online offerings at the fingertips of each customer.