In Star Trek, a micro-wormhole is a wormhole that is in an advanced state of collapse, and over time degraded and shrunk to a smaller size. It is a wormhole whose singularity has mostly dissipated, collapsing and leaving an extremely narrow passageway through subspace. In the episode "Eye of the Needle", the USS Voyager discovered a naturally-occurring micro-wormhole thirty centimeters in diameter which led from the Delta Quadrant to Romulan space 20 years in the past. In the episode "Pathfinder", Starfleet Communications was able to create a temporary, artificial micro-wormhole using the MIDAS array, to briefly communicate with Voyager over 30,000 light years away in the Delta Quadrant.
In physics, a wormhole is a hypothetical topological feature of spacetime that is fundamentally a 'shortcut' through space and time. Spacetime can be viewed as a 2D surface, and when 'folded' over, a wormhole bridge can be formed. A wormhole has at least two mouths which are connected to a single throat or tube. If the wormhole is traversable, matter can 'travel' from one mouth to the other by passing through the throat. While there is no observational evidence for wormholes, spacetimes-containing wormholes are known to be valid solutions in general relativity.
A study in 2005 by Chris Fewster, of the University of York, UK, and Thomas Roman, of Central Connecticut State University, US, demonstrated that if it were possible for humans to build such wormholes, they would probably be too small to fit spacecraft or even humans and could probably only be possible at the scale of Star Trek's micro-wormholes. The study analysed the proposal that wormhole throats could be kept open using arbitrarily small amounts of exotic matter - hypothetical materials possessing qualities like negative mass or being repelled rather than attracted by gravity. It is currently speculated that tiny quantum wormholes - at the level of quarks and electrons - may exist for short periods of time.
The micro-wormhole encountered by USS Voyager
Matter can travel from one 'mouth' of the wormhole
to the other by passing through its throat