Interdimensional rift

The interdimensional rift is a byproduct of the activation of the universe portal.

History
The rift is first mentioned near the end of "A Tale of Two Stans" when Ford tells Stan that while he and the twins stay for the summer he will remain in the basement to try and contain any remaining damage of the portal's reactivation.

In "Dungeons, Dungeons, and More Dungeons" Ford explains to Dipper that, as he suspected, when Stan reactivated the portal, the instability of its use created an interdimensional rift. He then hides the contained rift in a cabinet in his underground laboratory, being the only person aware of its existence, until he told Dipper, who he made promise not to tell anyone, not even his sister.

In "The Last Mabelcorn," the rift reappears in Ford's dream with Bill Cipher claiming he cannot protect it forever. It is revealed that if Bill obtains the rift, he can use it to physically enter the mortal world. Later in the episode, Dipper tries to protect the rift from Ford, due to his belief he is working with Bill.

In "Dipper and Mabel vs. the Future," Bill Cipher, possessing the body of Blendin Blandin, tricks Mabel into getting the rift, telling her it can stop time, fulfilling her wish of the summer never ending. Once Mabel brings him the rift, Bill smashes the containment orb under his boot, causing a portal to the nightmare realm to open in the sky.

Appearance
The rift itself has no definite shape, moving around on its own. The inside looks like space and glows a faint blue with various stars of differing; brightness, size, and shades of blue and black. It is contained in a snow globe-like container with caution tape at its base. The orb containing the rift is secured with pipe like supports, colored in a very dark grey, almost black.