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Fig - Theoretically diagram of a wormhole.

Wormholes and Black Holes are the theoretical solutions to Einstein's General Theory of Relativity. 

Also known as the white hole, they are time reversal of black holes. As black holes pull every matter, that crosses its event horizon, into itself, Wormholes eject all the matter from its event horizon. 

According to some predictions, the matter that the black holes draw into itself is thrown into another alternative universe which is connected to ours through wormholes. So, wormholes probably are the connecting between alternate universes. 

As wormholes connect the 4-dimensional space-time, Stephan Hawking and few others have proposed that wormholes can be theoretically used for time travel as well as space. But according to some theories, time travel into the past is not possible.

Source - https://www.physicsoftheuniverse.com/topics_blackholes_wormholes.html