If the Tesseract was in Asgard after the Avengers (Thor took it along with Loki) how come Von Strucker had it at the end of Captain America: The Winter Soldier? Even more at the end of Thor: The Dark World they said that they took the Aether to the Collector since they already had the Tesseract in Asgard. And didn’t want to Infinity Stones be together.
Von Strucker is not in possession of the Tesseract. He is performing experiments using Loki's scepter, which contains a different Infinity Stone. You can see the scene here.