The Tesseract (also called the Cosmic Cube) is a cube-shaped object said to possess unlimited energy. It is unknown when or who made the Tesseract, but over the course of history, it was used by both the Asgardians and humans. The Tesseract is one of the Infinity Stones. It represents space, and can be used with the Infinity Gauntlet, one of the most powerful objects in the universe.
Captain America: The First Avenger
The Tesseract was guarded by the Asgardians, the jewel of Odin's treasure room of relics, but lost during the last age of worship by men on Earth. It is found centuries later in the 1940's by Johann Schmidt (German SS officer and leader of HYDRA) in a box behind Nidhogg in a sculptured mural of yggdrasil world tree, in a church full of Viking warriors entombed within its walls. Schmidt and scientist Arnim Zola used the power of the Tesseract to create weapons against the Allies and the entire world. Captain America foils Schmidt's plan to launch an aerial attack on the United States, and Schmidt disintegrates into light that shoots into space when he touches the Cube. After Captain America crashes Schmidt's plane, the Cube is recovered by Howard Stark. At the beginning of the 21st century, Howard's note book with a drawing of a "hypercube" was found among his research and examined by his son Tony.
Thor
A book on Norse mythology featuring illustrations of the Norse gods shows Odin holding an object that seems to be the Tesseract. Years after being lost in the ocean, the cube is now in the hands of S.H.I.E.L.D.. Nick Fury, believing it to be a powerful energy source, recruits Professor Selvig to study it.
The Avengers
In 2012, after his defeat at the hands of Thor, Loki came into contact with the alien race known as the Chitauri, and their master, Thanos. For reasons which were unknown at the time, Thanos had already been seeking The Tesseract, and Loki promised to retrieve it from Earth and give it to Thanos in exchange for his army.
Through manipulation of the Tesseract's energy, Thanos teleported Loki to the Tesseract's location at the Joint Dark Energy Mission Facility, where he killed several S.H.I.E.L.D. agents and brainwashed Selvig, Barton and a number of other agents into following him using a scepter gifted to him by Thanos. He revealed his plans to use the Tesseract to enslave and rule over the human race, and quickly stole the aforementioned object and escaped. The energy from the portal destroyed the entire facility, while Loki took The Tesseract to an undisclosed location.
In response to Loki's threat, S.H.I.E.L.D. decided to re-start the Avengers Initiative, and swayed Bruce Banner, Iron Man, and Captain America to their cause. Fury also wanted them to trace the Tesseract as it was emitting small amounts of gamma radiation that only Banner and Stark would be able to trace.
While Loki was dealing with the Avengers, he ordered Dr. Selvig to begin working on a device that could utilize the Tesseract full power to create a portal that could transport the Chitauri army to Earth. Selvig and Loki later put their plan into effect and opened the portal above Stark Tower in New York City, which unleashed the Chitauri upon the city and a immense battle ensued. Agent Natasha Romanoff eventually used the Tesseract energy inside of Loki's scepter to reverse the effects of the device, shutting down the Tesseract and closing the portal. The remaining Chitauri were then defeated by the Avengers.
Following the invasion, Thor returned the Tesseract to Asgard along with Loki. Though his superiors were angry at this turn of events, Fury defended his decision to allow Thor to take the Tesseract back to Asgard along with Loki, saying that humanity simply wasn't at the level where it deserves the Tesseract power, much to the council's dismay.
