10 Big MCU Revelations That Didn’t Come From The Movies
5. Thor Used The Tesseract To Repair The Bifrost
At the end of Thor, the Bifrost - a method of instant transportation between the Nine Realms - is destroyed, and we're led to believe that it's impossible for Thor to get back to Earth without it.
But then, in The Avengers, he shows up on Earth anyway, with a throwaway line about "dark energy" serving as our only explanation. At the end of the film, Thor uses the teleportation powers of the Tesseract to whisk him and Loki back to Asgard, and then, in Thor: The Dark World, the Bifrost is magically repaired, and we never find out how.
This question mark was addressed in an MCU comic, where we learn that Thor and Heimdall used the Tesseract to fix it.
In the Thor: The Dark World prelude, we see Thor land on Asgard with Loki and the Tesseract. Then, he and Heimdall quickly use the power of the blue cube to "let the fires of Bifrost burn once more".
From this point on, the rainbow bridge was repaired, which explains its random re-appearance during the second Thor movie.