Interfold layer

From Memory Alpha, the free Star Trek reference.

An interfold layer is an unstable realm located in some regions of space. It is not contained within normal space or subspace, but in the confluence of the two. Objects inside an interfold layer are invisible to sensors. The plasma interference inside the layer requires recalibration and adjustments to subspace communications between normal space and the layer.

A region, where such a layer exists, was first encountered in 2373 by USS Voyager in the Delta Quadrant. It's existence was first detected when a probe was trapped inside it by an astral eddy that collapsed back into the layer. The probe became undetectable, but was still transmitting telemetry from within the layer.

While Tom Paris was gathering plasma particles, with the bussard collectors of the class 2 shuttle Cochrane, from the wake of an astral eddy, the shuttle was also pulled into the interfold layer by a collapsing eddy. This interfold layer was discovered to be the spawning ground of the astral eddies and found to contain thousands of small eddies. By piloting the shuttle to the eye of an erupting larger eddy, Paris was able to ride to the leading edge of the eddy and back into normal space. (VOY: "Real Life")