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. 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")

Images used to illustrate astral eddies in Star Trek very clearly indicate a phenomena similar to Black Holes, so an interfold layer could be thought to be a spawning ground where Black Holes can be formed. In actual astrophysics, Black Holes are made when very large stars die. When the star runs out of fuel for nuclear burning in the core it is no longer able to support itself from collapsing under its own weight. The star first collapses and then the outer layers rebound to form a supernovae explosion. What's left at the core is a Neutron Star or a Black Hole depending on the initial mass of the star. To form a Black Hole the mass left at the core after the explosion must be more than about 3 times the mass of the Sun. The star for most of its life probably needds to be between 50 to 100 times the mass of the Sun to eventually form a Black Hole.