Sometime in 2014 a Reddit post became popular using the gnome child from Runescape [0]. This was likely the first dank meme and a catalyst for what was to come. Using Google Trends [1] you can even identify that late 2013 a few people were already looking for dank memes, but it wasn't until late 2014 when memes started becoming dank.
The dankest they ever were was in October and November of 2016 - they've become less dank over time but the economists over in /r/memeeconomy keep their eyes out on /r/dankmemes for the latest and dankest memes to add to their portfolios.
It's a thriving industry with lots of karma to be had - it takes experts to identify a dank meme before it is ripe.
[1] https://trends.google.com/trends/explore?date=all&geo=US&q=%...
Now's my chance to ask a professional: when did memes become "dank"? Mid-2012?