Dark artifacts reduction

A few weeks ago I had 2,000 dark artifacts saved up from dungeons, now I have 90 even though I haven't used any. Seems like every time I finish a dungeon I lose my artifacts and I have to start over, anyone know what is going on?
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You can click on the top of the screen where it cycles your gold, battle chips, etc it will show you how much you have of everything there.
Ch1efster is correct. When you go to purchase an item with the Dark Artifacts it will show a screen titled "Insufficient Funds" with the amount you need to earn to buy the item. This is probably where you are seeing the screen that says you need 90 more.