Your dark chocolate might be a lot like milk chocolate, as reported by Quartz.
The Food and Drug Administration of the United States tested 94 brands of dark chocolate bars to see if they had milk. Only six of the bars listed milk as an ingredient. But it turned out that 61 percent of the bars tested had milk, and some had it in high amounts. Some of these bars had the markers “dairy-free” and “lactose-free,” while others read “vegan.”
Apparently, most dark chocolate products are produced by the same equipment that is used to make chocolate milk. No wonder some dairy gets mixed in.
You can get the full findings of the FDA here.