They are the riddles and riddler trophies.
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You unlock the Riddlers' hideout.
405 trophies i think
Press tab to open the map screen, you'll see icons at the top for the map, gadgets, and a question mark for the Riddler trophies. When you press that, it'll tell you how many you've found.
well after batman arkham city is arkham origins so yes there is
You need 400 before his hideout location is revealed. In the base version of the game, that's all of them, but if you've got the Catwoman DLC, there's an additional 40, but still only 400 are needed.