Antiverse

Overview
The Antiverse is what was believed to be on the other side of a Black Hole, until it was discovered to be a Body and not a portal. Right now, the Antiverse has been confirmed not to exist.

The portal theory
Assuming that all the black holes were a portal to an antiverse, upon entering in one of them, you would be sent out of a white hole in an Antiverse. Following this theory, the antiverse is just a specular image of Our Universe with inverted colours and inverted physical laws.