Whenever dark chocolate is brought up, we usually come across the word "flavonoid". Not everyone knows what it means, but it's what made dark chocolate "all the rage".
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Definition provided by Dictionary.com
flavonoid (flā'və-noid')
Any of a large group of water-soluble plant pigments that are beneficial to health. Flavonoids are polyphenols and have antioxidant, anti-inflammatory, and antiviral properties. They also help to maintain the health of small blood vessels and connective tissue, and some are under study as possible treatments of cancer. Also called bioflavonoid.
With that provided, it is known that dark chocolate is abundant in flavonoids. Check out the article covering Dark Chocolate Flavoniods in greater depth.