Non-wormhole based time travel requires either 1)unphysically large angular momentum densities, or 2) travel at speeds greater than light.
By contrast, numerous submicroscopic wormholes are generally believed to exist in every cubic meter of space. Enlargement of these requires "exotic matter". But if a fundamental scalar field such as the so-called Higgs field exists, it is possible to use it to prepare quantum states that correspond to exotic matter.
Source: The Physics of Stargates, by Enrico Rodrigo
It's nature and already incorporated into the universe. Humans just need to find it.
